The further (range) you can get the ball out there before it takes on it's curve-ball spin, the better your accuracy will normally be, with a smoothbore shooting balls. That is the theory, at least.
The more powder, the faster the ball, thus when patched, it isn't already spinning inside the bore before leaving as a loose rattling ball would or possibly be. You are actually shooing a knuckle-ball (base-ball type) - thus the faster it is traveling, the more accurate it should be.
In a .12 bore, 100gr. of powder with that ball should be just over 1,150fps, I'd estimate.
My 20 bore flinter does fine close-in at 25 yards (1 1/2" or so for 5 shots) with a mere 82gr. (3 drams) For shooting the long targets on the trail, and I do get a few of the 100yard gongs, I use 100gr. The 82gr. load does almost 1,200fps, while the 100gr. charge runs 1,375fps.