YOu have to measure both the bore diameter of your rifle, and the groove diameter. Get a caliper( borrow one if you have to) and measure the land to land dimension and write it down, then measure the groove to groove dimension. The land dimension is the Bore Dimension! It should be close to the size or caliber stamped on the barrel. If you have a .50 caliber barrel, the Bore diameter should be very close to .500". measure everything to the thousandth of an inch. The groove diameter, can run from .508-.512" and some will be even greater than that. The deeper the groove, the thicker the patch material you will need to use.
I recommend using an Over powder Wad( OP wad) on top of the powder charge, to seal the gases behind the PRB, and protect the patching from being burned with heavy charges.
If you buy pillow ticking. be sure to wash and dry the fabric in a washer/dryer to remove the sizing put in it to make it lay flat. Sizing is a starch like substance, and it tastes terrible, if you decided to use spit patches! It also makes it harder for oils, and lubes to fully penetrate the fabric, and that causes eratic groups. Measure the thickness of the fabric AFTER it has been washed and dried. You can use either a caliper, or micrometer. Expect the fabric to compress about half its thickness, so that is why a patch between a .490" ball and a .500" bore that measures .015" can do down the barrel fairly easily. Even a .020" patch will go down a cleaned barrel easily. The excess material gathers in the grooves.
Test only one change at a time, and use a bench rest to steady the rifle and your aim to eliminate as many human factors as possible during the testing. 5 shot groups will tell you more than a 3 shot group. When you have tried all your various patch combinations with powder charges, then consider trying a few .495" balls, and even some .500" balls with the patches to see if you can get even better accuracy. Try different lubes, but change only one thing at a time during our testing. ( for example, DO NOT change both the diameter of the Ball, and the lube you use at the same time for testing. You won't know if its the ball diameter, or the lube that causes a change, for better or worse in your groups. )
Yes, it takes a lot of time to find the best load for a particular gun. Some people are lucky enough to find a terrific load in a few minutes of shooting. Most of us spend months testing this and that to find out what works. The recommendations you can get here, once you tell us the caliber, the actual dimensions of the bore and grooves, the powder you are using, the barrel's Rate of Twist, the diameter of the ball you want to use, the patching thickness you have on hand, and the length of your barrel, and what kind of sights you are using on it, and the range you are going to test at, will go a long way to helping you reduce the time it takes to work up your load. The time taken is not wasted, because it allows you to become totally familiar with your rifle.