The distance from the center-front surface of the trigger to the center rear of the buttplate / recoil pad.
Some pretty good info here...www.ducks.org/waterfowling/shotgunning/index.asp
Remember, however, that these guys are speaking about sporting shotguns, and sporting guns are handled and fired differently than shotguns used for serious "tactical" (damn, I have come to hate that word!) uses. These tend to be utilized with a more "squared off" stance and are often shouldered during movement toward the target area. Many shooters also use body armor and sometimes tac vests, which adds to the bulk. Thus a LOP that works well on the skeet range may be too long for more serious purposes.
13" or thereabouits tends to work fairly well for me, but in my experience, too long is MUCH worse than too short!