In the world of high dollar custom guns, an adjustable stocked "Try-gun" is used to get the measurements needed for an individual shooter, and the stock is built to those figures. For a home project, the easiest method might be to find the gun that fits you best, and copy the measurements. The important ones being pull length, drop at comb, drop at heel, pitch,and cast-off, if I remember correctly. ( No, I never had one built, but I look at the manufacturers numbers when shopping for a new gun.)