When shouldering your conventional sporting stock shotgun there should be two fingers width between the back of your thumb and your nose. The reason for this is obvious, you don't want recoil bloodying your nose with your thumb. This holds for whatever your using the shotgun for. The proper LOP for you is what ever length permits this two finger width. Now if you opt for a pistol grip shotgun for your turkey gun you can get away with a LOP one inch shorter because your thumb is not placed over the top of the grip.