what's the correct way to determine the optimal length of pull (pertaining to rifles, in general) for an individual?  how does it translate to inches?

Hold your rifle in your shooting hand, pointed towards the ceiling with the butt resting on your bicep, and you finger on the trigger(arm at a 90 degree angle).  Does it fit?? Is your arm too long, or do you have to reach for the trigger.  Not real scientific, but close.

Quoted: Hold your rifle in your shooting hand, pointed towards the ceiling with the butt resting on your bicep, and you finger on the trigger(arm at a 90 degree angle).  Does it fit?? Is your arm too long, or do you have to reach for the trigger.  Not real scientific, but close.

What he said.  Crook of arm to crook of trigger finger.

so it should be the same length as your (inner) fore-arm + hand (trigger finger)?  

...to the center of the first tip of your trigger finger (middle of the fleshy part).

Hold the gun in the standing firing position. Grip it in the normal firing position with stock on you shoulder. Now pivot the buttpad out so the stock is parallel to your forearm. DON'T change the grip of the firing hand! Next, point the muzzle up at the ceiling. Grip the gun tightly with the firing hand(if it's an AR just squeeze the pistol grip). Keep your forearm pointed straight up at the ceiling, and your biceps/triceps are parallel with the floor-Your arm is a rigid "L" shape.
There will probably be a gap of 1-2.5 inches between the buttpad and the inside corner of your elbow. That is how 'short' the gun is for you. If your arm won't fit without readjusting your firing hand's grip, the stock is too long for you. -If you're over 5 ft 10 or 11, most factory stocks will probably be short for you.