Your not going to get it figured out unless your willing to spend a few bucks..... For starters I would buy a powder measure and start at 70gr of loose powder, Pryrodex RS or Triple 7. Stick with the 250 gr Powerbelts for now. Shoot a round, run a "damp" patch with a 50/50 mix of windshield washer fluid and rubbing alcohol, followed by a couple of dry patches. Keep shooting this for a while and see what your groups do. Next try upping the powder to 75gr, up to 80gr, up to 85gr and so on until your group gets better. But try the same powder gr a few shots until you up your charge. Different muzzle loaders shoot different, you have to work up a load your Optima likes. Your Optima might group well with 85gr, 90, or 110. You have to be willing to take some time and see. The Power Belts group good with my CVA Wolf with 2 50gr Pyrodex pellets and a 245gr Power Belt. And I'm still playing around with the loads. I've seen identical guns group different with the same load. It takes time. Just be sure not to load heavier than the recommed limits of the Optima. More powder is not always the answer. I'm grouping 2 inches at 100 yards and I'm going to try some Barnes MZ Expanders in 250gr and some Hornady bullets.
** I should add that you should maybe consider setting your target at 25 yards to start off with, GOOD LUCK!!**