With normal rifle, what DP and bgf say works well. With vary narrow grooves and wide lands (opposite of ideal) you might have to start with a ball .010"under the bore diameter.
So- the first thing to do is look at the rifling type. If equal or wider grooves than lands, .005" is right - that's easy. Next is to measure the bore diameter to the nearest thousandth of an inch. Subtract .005" and that's the ball size. Most normal barrels from regular suppliers can be trusted for size, ie: their .50 needs a .495" ball, a .45, a .445" ball, that sort of thing.
Next is patch material - denim works for us all - but some might prefer linen or ticking. I just can't see paying over $20.00 per meter for linen, besides the stores don't carry it in thick enough bolts. The .005"/under ball needs a .020" to .022" thick patch. 10 or 12 ounce denim works for us - all of us. Thus finding a load is no more technical than changing powder charges while shooting at 50 yards, minimum for 'testing' loads. I prefer to zero the sights at 25 after finding a load, then I know it is virtually zeroed at 50 & 4" low at 100yards - roughly, within fractions of an inch. I zero at 25 as accuracy of holding is more easily accomplished and the results come in quickly. No wind or other distractions to muddy the sight picture. Then, check the zero at 50 yards and further.
That is a standard recipe that has worked in every barrel from 1973 until present- no exceptions. The only thing to do is to make sure the crown is nicely smoothed and try different powder charges and slightly different patch thickness. Forget about .010" to .015" patching - it isn't even any good for cleaning - 1 exception - if you are shooting a ball larger than the bore, .015" might be the best for your gun.
I have found that a ball .005" under, along with 10oz. denim works in all guns for target-type loads, but some prefer thicker patching for heavy hunting loads, especially in smaller bores, like .45 or .50 with higher pressures than the larger bores.
One other thing, if a .020" to .022" patch is difficult to load, something is wrong. The crown is too sharp or not enough lube.