Ken, I made a spreadsheet calculator that makes these sorts of calculations easy. In your case, it would be helpful to know the current length of the barrel because the taper depends on length as well as width, and each inch is different in a tapered barrel. The first inch at the muzzle will be lighter than the last inch at the breech.
Anyway, if you remember your junior high algebra and geometry, you can pencil out the number or give me the length and I'll do it for you.
Brent