The easiest way I have found is to determine oal is take a spent case, not sized or anything, shove a projectile on to the end of it, then push this into the chamber until the bullet is seated all the way. Then measure that case, that’s your max oal for that chamber and projectile. Back off your loading oal the desired amount from the max, example .015 below max oal.
Yes xl40 does longer rounds, that would be the one you want.