Mike's correct, Paul's legally right :blah: The better answer is to first mike the bore, then select a ball, patch, powder combo that works for you. .530, or .535 is a good ballpark, but RB's .520 will also work with the right patching/powder. Everytime someone, generally a newb asks for the best load for a bore size, there are a gazillion answers, and that's because with blackpowder guns it's a matter of choice, weather, likes, dislikes, and folklore. That's the fun of this hobby.
Bill