cutting the patches at the muzzle makes sense, but it can create extra wear on the muzzle. Cutting them ahead of time certinly saves fumbling for a strip and patch knife every time you load. Some folks think it is easier than fumbliing for a pre cut patch.
mathmatically, a patch needs to be slightly larger than half the circumference of the ball so it creates a seal around the edge where the ball rides the bore. Remember the formula for the circumference of a circle??? pi x diameter. Assuming a .50 ball x (pi) 3.1417 = 1.57 inches. You need a patch that is at least half of that, or slightly more than 3/4 of an inch. Now trying to center that and start the ball with exact placement would be difficult. So some overlap is required. 7/8 inch diameter patches would still be a problem, one inch patches will work great. Now cutting all those circular patches can be a pain. I have often used square patches when loading, and unless the square is drastically over size, I found no difference in hunting accuracy. Precision Target accuracy is probably affected, by the extra corners of the square patches. To me, hunting accuracy is a 5 inch group at 100 yds. Precision target accuracy is cloverleafs or less at 100 yds.