Mr. Leggs explanation is quite accurate from my experience. There is one other dimension that also comes into play and affects how tightly the shooter may be able to grip the gun while placing the pad of the index finger on the trigger. That is the diameter of the wrist on an English Straight Hand stock. A very slim wristed stock will be harder to control than a thicker one, especially if the shooter's hand is at least medium to large in size. A thicker wrist diameter or one with more of a diamond shape fits most hands better. Combine a slim wristed stock with a L.O.P that's even just a little bit short and the result may be a bruised middle finger.

"I hate rude behavior in a man. I won't tolerate it" - Capt. Woodrow Call