Two cents more thrown into this discussion shouldn't hurt. When I'm trying to help someone determine their proper length of pull I watch them mount their gun (assuming they are starting from a low gun position). If they can't bring their gun to their shoulder without first pushing it away from their body, then the present LOP is too long. We work backwords from there.
Disciplines like American skeet & trap can accomodate a slightly longer LOP because they allow for pre-mounted guns. All FITASC and most game shooting is done with an unmounted gun, and you generally don't have time to suffer with a gun that you can't immediately bring to your shoulder.
