"Action" is the term used to describe how far the strings are from the frets.
This measurement is usually taken at the last fret by measuring from the top of the fret
to the bottom of the string. You can use a feeler gauge but using a simple mm rule is just
as effective, in fact I don't use feeler gauges for anything ;) Many times you will hear
action measurements including a measurement at the 12th [or some other fret]. Measuring at
the 12th fret assumes every necks bow profile is the same and they never are. The bow
profile of your neck will ultimately dictate the kind of setup that works best for it.
[Bow is the amount of relief or "frontbow" the neck is dialed into, bow profile
describes where the relief is] Some necks will show bow only from the 7th to the nut, some will show a very long smooth
even bow throughout the whole neck, and many other profiles. I measure action at the 24th,
but only to get a number to advise others, I would never setup a guitar with a mm rule,
each guitar is unique and has it's own peculiarities.
While on the subject of buzz, lighter gauge strings will have more buzz than
heavier, and different brands will buzz more or less than others. I use
D'Addario's which are a very pliable string but also buzzier because of this. A
set of DR Tite Fit is much less pliable and will not buzz as much given the same
action and setup.
Silly Low - Some like the neck perfectly straight or with the slightest amount of relief, and the action right on the frets. Perfectly straight neck and extremely low action will give you playability that's unparalleled for some people, a very low resistance to fretting, but there are sacrifices. The lower the action the more the strings are being choked by the frets. It has a way of disguising fret buzz by smoothing it out. On a medium action you'll hear the buzz quite well as the string is usually just pinging off of one or possibly two frets. On silly low the string is pinging off of so many frets it's not as apparent as "buzz", but what all that contact with the frets is doing is robbing your notes of the full tone that string could offer if it was allowed to breath. Processed signals can virtually hide the choking, but if you ever plug into a clean channel you'll notice right away. You can get away with silly low if you've got an extremely light pick attack. Typically in a setup this would be a perfectly straight neck to .1-.3mm of neck relief, and action height at the last fret of 1.5mm or lower. Not good for playing big bends without impeccable fretwork and a flatter radius fretboard, and even then the lower you go the less you'll be able to bend without choking. At 2mm you can get a 3 step bend but 5 steps you will not. You will also have to be more precise in muting as it's much easier to "mute" a string into the frets and create noise. I love the playability of it when I get to set one up like this, but I loose a little feel of the string when it's that close and prefer just a little better grip I feel I get at just a little higher action. Your Mileage May Vary [YMMV].
Standard Low - Adding just a little more neck relief and raising the action will give a cleaner tone, longer sustain, and tolerable buzz with a medium/light pick attack. This is usually the typical setup for guitars I ship. Good clear tone on the majority of the neck with and a light fingering feel. Typical is .2-.3mm of neck relief and 1.6mm string height on the low E, and 1.5mm on the high E at the last fret. This gives me the effortless playing of silly low but just enough tension on the fingertips to get the feel of the string, especially for bending, and muting becomes a little easier. As I mature I have grown to like this setup better in the middle range. Tastes change, and you should set your guitar up to your taste.
Low - Continue raising the action will get still cleaner tone, and lighter buzz with a medium pick attack. The higher you go the more tension your fingers will feel, and this creates more control of the string for me. I typically keep the relief down in the .3mm range to reduce the amount of "boing" from the slightly higher action in the 4-12th fret range. Typical is .3-.5mm of neck relief and 2mm string height on the low E, and 1.8mm on the high E at the last fret.
Medium - You can still add a little neck relief but after a certain point more relief just isn't an advantage. Raising the action height is and will continue to clean and fatten up your tone and increase the feel of the strings under your fingers. Medium relief is .5-.6mm of neck relief action of 2.5mm on the low E and 2-2.5mm on the high E.
High - Lots of frontbow and high action will give you a very clean playing guitar with full tone, and lots of extra calluses. However much neck relief you want and action on the low E at 3mm or more, action on the high E over 2.2mm to as high as you want.
There is another school of thought that the higher the action the less relief the neck needs since the height of the action itself produces little buzz. Relief with high action is more about keeping the action more even the length of the neck and not as much about buzz. Like anything in a setup it's all about the compromise and the preference of the individual player.
The wound E is going to vibrate in an ellipse that will vary by how hard it's struck. Strike it light and it will have a small ellipse, strike it hard and it will have a much larger ellipse. A higher gauge string will vibrate in a smaller ellipse because it is strung "tighter" than a smaller gauge string to get the same pitch. Your action should be compromise of how much buzz you can take, the feel you want from the strings, and the tone you want to produce.
Setting the Action - Action is my last adjustment in a setup, after neck relief and the trem angle are set I will dial in the string height. With it approximate, I will then break and adjust the nut height since this is dependent on everything being in correct setup to get as low as possible without open string buzz, then do a fine tune on the action after the nut is set for the final adjustment before playing the guitar to determine if it needs any further tweaking.
Adjusting the action varies by the type of guitar but since this is an Ibanez site you'll find Ibanez directions. ;) Final action adjustment is the last tweak to make and should be done when the neck relief is correct [for you] and the trem angle is correct. After it's set make a final check of the setup to be positive nothing else has changed. Once action height is set it should never need adjusting, the only things that will change it are the natural changes your neck will make as it warms/cools/humidifies/dries, and the trem angle which will change for the same reasons, as your neck moves, and your body swells and shrinks. Adjust what is changing, don't just tweak the string height as a quick fix instead of tweaking what has caused the action to change [although between string changes even I'll go for the quick fix over a re-setup].
It is perfectly fine that the bridge is at an angle!! [the treble side lower than the bass]. Wound strings need much more room to oscillate than the fine treble strings. You can run treble action much lower than bass without buzz and is perfectly fine to do.
Floating Bridge - Edge, Lo Pro, Edge Pro, and all of the double locking Floyd variants are adjusted by raising and lowering the trem studs that the knife edges pivot on. Edge and Lo Pro guitars - There is a small 1.5mm Allen adjustable set screw inside the studs on all Edge and Lo Pro equipped guitars [ONLY Edge, and Lo Pro guitars] that must be kept tight for the best tuning stability. Never try and loosen it with the allen wrench but use the 4mm Allen to back out [Lefty Loosey] the studs a hair to free the set screws. If raising the action raise it until it's correct and retighten the set screws. If lowering the action use the 1.5mm Allen to back out the set screw enough to allow the bridge to be lowered. When correct retighten the set screws. Important! After you've tightened the sets using the 1.5mm Allen use your 4mm Allen to take the slop out of the threads between stud and insert to really lock the threads together and give a very solid fulcrum for the bridge.
The amount of slop in the threads will depend on the mate between the stud thread and insert thread. They won't all be the same. Some will take a 1/4 turn [especially if it's a stud mod for EP], and some will take just a hair of rotation to lock. Knowing which will let you know if you can set the final action first, or have to leave it a little higher for the tightening to finish setting the action. But your studs aren't locked until the slop is out of the threads. You're just trying to get the slop out, you are not trying to weld the 2 pieces together so don't try grinding everything together or you'll either break the head off the stud or spin the stud anchors in the body, use good sense when applying torque, but you do want the threads locked together.
All other floating bridges - Just use the correct size Allen wrench to raise or lower the studs. Adjusting the string height to a large degree will alter the tuning slightly, which can alter everything. If you are making extreme action adjustments be sure to retune, readjust the trem angle, and then recheck the action. Yes, you can adjust the studs with the strings at full pitch without fear of damaging the knife edges or the studs.
Vintage Trem and Fixed Bridge - Adjusting the action of these types of bridges entails adjusting the height of the 2 adjustment screws on each saddle. Each string is adjusted independently so it is important to keep the bridge radius in sync with the fretboard radius. Do this by measuring each string at the last fret. With this type bridge I always like to keep the same radius to the saddles themselves so they're more of a nice arc than a stepped feel across the bridge.
Gibson Type Stoptails - These are all generally adjusted by raising or lowering the mounting studs for the bridge using wheels built into the studs. On a Gilbraltar type you have to loosen the screws on top of the bridge to be able to raise or lower the bridge using the wheel adjusters. It is advisable to slack the strings to decrease the tension on the bridge to make these adjustments. Tune and check the action, repeating as many times as necessary to get correct.