I agree with you. Try the .010 patchings. The math is right, and they should work well for you. If you do get burned patches, use an over powder wad to protect the patch when you load the gun, and you should be fine. If you went to a .595 ball, that would give you .025 in extra size to divide between the two sides of your ball. That would give you .0125" for a patch thickness- pretty close to that .012 patching you say you have. But that would also be a fairly snug patch, and I know you like to shoot without cleaning a gun between shots, so that might not be the combination for you. If you use the .595 balls, stick with the .010 patches, for an easy load. The balls will bump up to size of the bore that much, on firing, and I don't think you will lose any accuracy. Again, use a overpowder wad when loading these smaller balls, or if you find any evidence of burned patches on the ground. In a smoothbore, the patching just holds the ball snuggly in the barrel, so it doesn't move forward and become a bore obstruction when you fire the gun. It doesn't impart spin, as with a rifle. It really can't " Grab" the ball, either, until the ball is expanded, trapping the fabric between the soft lead ball and the wall of the barrel. So, don't ask it to seal gases behind the ball. Use that ovepowder wad to do that job, and leave that thin patch to just keep the ball in the barrel. It will protect the barrel from leading, which you indicate you have no interest in having to remove. I don't blame you. Even with modern solvents, it takes elbow grease to get lead out of a smoothbore.