OutlawSix:
My suggestion is to surf the web-sites selling British double-guns in .465/.470/.475 and make a list of the weights. I suspect you will find everything from Holland's "New Lightweight Style" at 9 1/2 pounds, up to about 12 1/2 or 13 pounds. You could calculate the 'mean', or better still the 'mode', and see if it would suit your single-shot.

Other than that, I would agree with Westley_Richards' previous post and go with 11 pounds. I sometimes carry an 11 pound .577 double all day, and don't find it too much of a hardship.

If I was to offer anything further, it would be to take great care to concentrate as much weight as possible between the hands, which means no mercury in the butt, and make very sure the stock fits you. Good handling and stock fit will do 10 times more for FELT recoil reduction than any gadget in the butt (IMHO).

Best of luck with the project!