Simple way to calculate patch size: Take a ball with patching and start it flush with the muzzle. (no powder of course!) Cut patching at the muzzle. Drill a hole in the ball, insert a screw, pop the RB out. I would then make all pre cut patches about 1/4" smaller than the muzzle cut one. Thickness: I like .018, to start with and then see what the shooting results are. Go from there. Your patching needs to cover the sides of the ball perpendicular to the bore and then a tad more. You are trying to seal the gases and have the ball follow the rifling.
For most reproduction rifles, many shooters use a ball that is .010" (0.254 mm) smaller than the bore and a patch that is .015" to .018" (0.38 mm to 0.457 mm) thick.
Greetings Furrier. I've enjoyed all the different comments about patch size and what's the best, and just for something to do I was making square patches out of Pillow ticking, about an inch and 5/16 square for a 50 caliber RB. So I went out shooting with a friend shooting a 45 caliber RB. He was using square cut patches that were 2 inches in across. I thought holy smokes that will never work, and his accuracy was as good or better than mine with no problems. He sure didn't have to spend much time centering the ball to keep cloth all the way around. It fit tight enough in the bore that he didn't have to worry about pulling it off the powder, So thickness is your big concern as far as I can tell.
