When folks say - action low, or action high... it means nothing. Its like long and short hair. Compared to what, or who?
Now... should someone say, my hair is six inches long, or my action at fret 12 is 3.5/64ths of an inch, then we know exactly what is being said.
First off - only need to shim the neck if the bridge wont go low enough, or the pickups cannot be set lower and they interfere with the action.
Second, tune to pitch, ALWAYS, then check the relief at fret 8 while depressing fret one and fret 21 (or 22). For most players... all that I know... about .008 to .010 is the relief that works. A totally flat neck tends to buzz at the low end, close to frets 1 through 3ish. Too much relief will fret out and buzz at the high end.
After the relief is set, its good to know there are mathematical limits to action height, depending how much you want to bend the string and where. No matter what, given a low enough action, things will fret out while bending. Its math, geometry, you cant escape it, but you can work with it.
Everything I mention should be checked via magnifying glass and machinists rule, measuring from the bottom of the string to the top of the fret.
For 12in radius necks, at 24-3/4 scale, you can set the relief to .010, and the elevation at fret 12 to 3.5/64ths of an inch, and 4/64 of an inch, respectively at the low and high E. Graduate the rest of the strings in between.
For tighter radius, you'll need to raise the strings a little more, as much as another .5/64 of an inch. All of that should give you at least two steps of bending, maybe three, depending on how heavy handed you are. Need more bending room? Raise the action a little more, so it doesn't fret out. All of this fretting out business is at frets 14-20ish, where you bend up and it chokes the note. The string is essentially crossing over the frets due to angle and the "hump" of the fretboard radius is in the way.
Folks say... my action is all messed up, its too high, low, whatever... and they think... its the guitar, and it needs a "setup". Nada. Either they changed strings (changing the relief due to different tension) or the weather got dry or wet, and the neck shifted a little. Either way, its the relief, and only the relief that needs adjustment at that point, to get the elevation back in where it needs to be (assuming nothing else was altered). When the neck gets into a humid condition, the fretboard grows longer and you get more backbow tension. In dry weather, it gets shorter and you get less backbow tension. The truss rod needs minor adjustment to compensate. Here in Fl, we run AC all summer when its hot (and wetter), but in the winter we open the windows. In the AC, its dry inside, very dry, and fretboards shrink. When the windows open, its more humid, and the fretboards grow. So... I adjust my relief for the winter humidity, and live with a slightly higher action in the AC months (or humidify the case).
When the relief is set too great, then your high frets will buzz... why? Because the player compensates for the high strings by lowering his bridge. WRONG. Now the angle of string to neck is all out of kilter at the high end, and you get more buzzing and fretting out. Again... the KEY to good setups is knowing what good relief is. You should have about as much room under the string... from bottom of string to top of fret... as your high E string is thick, or maybe... just a tiny bit more. Use a strong magnifier and maybe even a bit of E string to check. Having a buddy help is good. Dont MASH the frets. Press lightly... at fret 1 and the highest fret.... and see how much room there is under fret 8. When you make adjustments, go easy, and let it sit a while before doing any further adjustment. Not days... but a half hour or so. Just play it during that time, then recheck it, making small adjustments. If you go for a big adjustment, then you might set up tension which will continue to shift things even after you put the guitar away for the day. Small adjustment, and several of them, is better.
Ok, nuff on that... go do it.
