I just measured my Pedersoli "Tryon". It is a back action rifle but its profile is very close to a Hawken. It is like the bottom rifle in the first post. Here are the critical measurements from MY RIFLE!
1. The diameter and the two inches windage over the hammer and trigger guard is 12 inches. This was measured 18 inches ahead of the butt.(the barrel is 32.5 inches)
2. The diameter plus windage at the back sight is 8 inches.
3. The diameter plus two inches at the muzzle is 6 inches.
If you leave an inch for the seam at the muzzle, your pattern should be a bit over 51 inches. You can trim the case at the butt after it is finished, but it is harder to lengthen. The pattern must be 12 inches across for at least 18 inches from the butt and can taper from there to six inches at the muzzle. If you want to be a bit cautious, add a bit to the measurement at the muzzle, but make it less than 8 inches. These are my suggestions. Perhaps another Pard can go measure their own Bundook to second guess me. I won't mind!