Okay, I finally got a day of clear weather to test out the different charges.
I put together prepared charges of 150, 200, 250, and 300 grains of Goex Cannon powder. I was shooting at a target 50 yards away.
150gr Shot 1 - couldn't locate where shot landed
150gr Shot 2 - round keyholed in target board on lower right
200gr - round punch through board above targets right shoulder (left side) and either it wasn't tumbling that badly or it just happen to hit at the right point in the spin, since the hole seem a bit ragged, but otherwise round.
250gr - I mislaid the gun, so the round hit way right, but my spotter (a friendly shooter who happened to be on the range at the time) said it seemed pretty straight. The gun still jumps. so this is probably too big a of a charge already.
300gr - Way too big a charge. Gun jumped and the round overshot the 50yard berm and impacted about 10 feet up on the 150yard berm (trees and a large hill past that, so we were safe even if it went over that one).
So, in summary, it looks like the 200 grain charges are the gun's sweet spot.