Well I am a NRA/USAS Certified Advanced International Shooting Coach, and the way we determine length of pull is put the butt in the crook of her arm (bent elbow) and measure to the trigger finger in firing mode.
Great! Get it simple enough for everyone to remember and forget the part about it actually being reliable.
I just measured by your method and arrived at 17.5". My actual preferred LOP on rifles is about 13.75". And for shotguns I add about one to two inches. That leaves me almost two inches shy of your number.I have seen many folks whose measurements are even farther off.
Obviously the OP is asking about rifles but that foolish number has been held as gospel by shotgunners as well for years and it is useless. Yes, many CAN shoot with guns that match their arm length, by why not use their Astrological sign, too? It is about as relevant.
