I have been shooting a load that thumb starts in my bess for nearly 25 years with good accuracy. IMHO, all the patch does in a smoothbore is hold the ball centered on the bore, and you don't need a patch/ball combination that needs a short starter to achieve good accuracy. IMHO, too many folks want to use a too large ball that requires a thin patch, which IMHO, harms accuracy, not to mention ruining the pure joy of shooting smoothbores.
Lets say, hypothetically, that your bore is a true .540. And lets say you are trying to shoot a .530 ball.
.540-.530= .010/2=.005 patch thickness. IMHO, a .530 ball with a .010 patch won't go down without a mallet.
.540-.510=.030/2=.015., so a .510 ball with a .015 patch should load fairly easily, because the .015 patch has enough crush to allow fairly easy loading with a tight enough fit to shoot well.
Each gun is an individual to itself, so ya' gotta try different loads to learn what it likes.
I suspect that a .500 or .510 ball with the proper patch thickness, lube, and powder charge will shoot as accurately than you can hold it at 50 yards.