The combined thickness of the patch (doubled, of course) plus the diameter of the ball should be about equal to the GROOVE diameter of your bore. If the patch/ball combinationexceeds thisdiameter, and you can still start the ball without deforming it in the process, accuracy should be acceptable (provided, of course, that the rifling twist is appropriate for a round ball and the powder charge is reasonable.) The patch material may compress somewhat on loading, and if it bites into the soft lead of the ball, so much the better. The patch has to grip the ball firmly to cause it to rotate on firing.

If you think of the patch as a "cloth sabot" it might be easier for a person with inline shooting experience to grasp what's going on here.