One of our BC shooters from the Island (Vancouver Isl) showed up at Hefley Creek Rendezvous one year, with a Purdy .52 belted ball rifle.
As with 'back in the day', he also shot with with a cloth patched belted ball. I do not know what the tolerances were, but would expect it allowed
a substantial patch of at least .015" thickness. I am sure he used at least that thickness of patch.