If it's a new GPR, I can offer these details from my own.
When new, use some carb cleaner or brake pad cleaner on cleaning patches to get rid of ALL the packing grease Lyman has put in there. It's tough to get out completely with any other solvents, but easy with one of those cleaners. And you have to get it out for good results.
You might find that the sharp muzzle or sharp lands will cut your patches at first. If so, figure on shooting 100-200 rounds to smooth it out, or take matters in your own hands. I laid a piece of 400 grit emery cloth over the muzzle of mine, pressed on it firmly with the ball of my thumb, then rotated half a dozen turns. Presto, sharp edged gone, patch cutting mostly stopped, and good accuracy.
The sharp rifling will smooth out with time, but lotta guys like to use a small piece of those green Scotch kitchen pads with their jag to speed it along a little. Takes 50 or so strokes, but worth it from what I hear. I've always chosen simply to shoot my GPRs. More fun, if not as accurate right away.
Both my 54's wanted to start out with .530 balls and .015 patches. Using that ball with .018 patches prooved too tight for my tastes. After a couple of hundred shots I noticed balls starting was getting pretty easy, so I switched up to the .018 patches. Presto! Exceptional accuracy even if they weren't so hot when the rifle was new.
I have several thousand rounds through the rifles now, and starting is pretty easy once again. I'm tempted to move up to .535 balls and .018, but haven't got there yet.
Long and short, if you go with the .535 balls while the rifle is new, you're likely to need really thin patches on the order of .010. I'll almost guarandamtee that if you do so, you're going to run into patch cutting problems.