My old Ped bess mics .748 and I have had the best accuracy with .718 ball cast out of ATM (anything that melts), and .018-.020 patching. I haven't' found that patch thickness matters all that much, as long as there is sufficient crush to the patch.
Generally speaking, smoothbores shoot best with a ball dia about .020-.030 smaller than the bore dia. though even smaller balls can be used with good accuracy, as long as the patch thickness is sufficient.
A good way to determine a beginning patch thickness for your bess is to subtract the ball dia from bore dia, divide by two for the patch thickness.
For example,
.743-.715=. 028/2 = .014 patch thickness. Again, that is a place to start. IMHO, an .018 or .020 patch might provide better accuracy and still load easily, but maybe not.
God bless