Each gun is different, and you simply have to keep messing with them and changing the variables until you find the right combination that works for your gun. You might find that 85gr of FFFg with a .535 cast roundball over a .015 pre-lubed patch is dead on for you. I could take that same load and put it in my GPR, and might not be able to keep a 12" group with it! This is called "working up a load".
You should start with what you have, and slowly add to your charge say in 5/10gr. increments, and shoot groups and see if they shrink or grow. Then, try a different patch and do the same thing. Then, try it with FFFg (keeping charges about 15% below those of the FFg). Then, if you can try different powders, you should try Goex, Swiss, etc. Try different lubes. Try different roundballs. Try swabbing the barrel between shots. Keep track of what you do, so that when you do find the magic combination, you can remember it.
If the gun is brand new, you might need to just shoot it a bunch to get the newness out of the barrel. Some folks lap their barrel with steel wool wrapped around the cleaning jag.
This process is part of the fun of shooting. You get to do a lot of shooting, and you get a good feeling of satisfaction when you figure it out. I still haven't found the magic combination for my GPR, but I'm working on it. :front: