The Wulff double people have 48 measurements they take to find your LOP. The arm measurement you mentioned gets you in the ball park, but it is more complex than that.

It is not only your phyical dimensions but your shooting style that is important. Moreeover, as your shooting style progresses, your best LOP can change. It can change 1 1/2" even.

The best way (if you are satisfied with your shooting style) is to practice mounting guns of the same design as in which you are interested. If the gun catches on your clothing as it goes up to your shoulder, it is too long. If when shooting the gun, your nose attacks your thumb, it is too short. The problem is you must do this practice a lot but not all at once because you will subconsciously alter the mount.

If you will be using this gun is both cold and hot conditions, the thickness of your clothing might cause a problem. Some manufacturers make pads of varying thicknesses which snap into place so that you can alter the LOP to suit conditions or different shooters.