Run some tests with everything from about 80-150. 150 should be well within safe limits. However, you may find that accuracy drops off rapidly past a certain point. I have been shooting BP for many years, and I have always found that mid-range loads are the most accurate by far. the slight decrease in velocity and ft/lbs will more than be made up for by the increase in accuracy. You will likely find that something between 90-120 is your most accurate. I have 3 in-lines, and I never use more than 2 pellets in them. Nothing to do with recoil or cost, it's simply because they are a lot more accurate at 100 than at 150. Also, in several of my percussion lock 45's, the most accurate load is 70 gr. of ffG behind a 250 gr. maxi-ball. Not a train-stopper, but it will stop any critter in North America, and i don't often have to shoot at trains anyways.
One thing, I don't know how familiar you are with BP, but it, and substitutes are measured by volume, not actual weight of charge.
