As far as shooting low, you may want to up your charge. Remember that the standard load for the Bess was from 140 to 160+ grains. Mine is much more accurate with heavy charges. 1fg works best in my Bess, but 1fg likes a heavy charge. For the very light charges/under 100 grains, then 3fg or 2fg probably works well. But old Bessie really "wakes up" with heavy charges of 1fg. Again, that's where I get best accuracy.
I think the .690" ball works best. Just use heavier patching. Denim makes a good patch for the .690", (in my barrel) and if I "chew" the ball a .023" patch works well. It also fits paper cartridges better, if you don't like grocery bag paper, and want lighter just use sammich bag paper.
Hope that helps.