I'am with the Virginiarifleman,I'am 5'11" and my Lop is 14.5" ,, long arms,, I took a yard stick rested it in the crook,( elbow) with my arm raised at a 90 degree and curled my finger , picture your arm in the drawing to be "B" This works for me,however as I pointed out,,my arms are long for my over all height. This also keeps the rifle/shotgun from pushing my thumb up under my nose,,
And if you make it to long ,,he can have it resized,,to short,, not so easy to lengthen it!!!