The quote is from "Colonel" Glenn Baker of Woodcock Hill (Thom. Bland "owner" and distributor in U.S). and the words were spoken to me. The 90 degree included angle to the rib would give you the neutral or zero pitch that G. I. Joe mentions. 83 to 85 degrees included angle, with the butt in full contact to floor and the breech against the wall means you'll see some daylight (2 or 3 inches) between muzzle ends and wall. You see it's all about house carpentry and horse shoeing--or once was.
I agree that you should see how the pitch thing works out for you with the long stocks by adding quarters or tapered shims or whatever before you cut. May reduce cheek slap and muzzle flip to a certain degree but really you're only trying to achieve full contact with the side of the pectoral muscle in the shoulder pocket so the butt and comb remains in the same relationship to your cheek and eye (rear sight) in case you might want to fire a second shot after the first. I am not a gunfitter and I don't play one of TV (or here on the internet) but you did ask so that's my .02.
jack
