We have an old Tennessee rifle in the family that came with the original mould. Sorry, but I don't have any pictures, and I don't even have the rifle in my possession at this time, although it is still in the family. Anyway, this one had an unusually large bore for a Tennessee rifle, somewhere around .53 caliber. When I was young and foolish, maybe about 16, I shot this rifle a few times with light to moderate loads, using balls run from the original mould and patched with old ticking salvaged from a discarded mattress. I specifically remember it was easy to load, without a short starter, and it shot very well.
In Small Arms and Ammunition in the United States Service, author Berkeley R. Lewis reported that the US rifles which were intended to be loaded with patched round balls used balls .015" under bore size, e.g. a .54 caliber rifle shot a .525" ball. This is corroborated by some of the other references I have found.
In The Kentucky Rifle, Captain Dillin stated that many early hunters had two moulds for each rifle, and they were in the habit of carrying two sizes of balls, although he indicated one would be under bore diameter, for shooting with a patch, and the other somewhat larger, for a quick reload without a patch.
I imagine a lot of old rifles were separated from their accoutrements as they were passed down through generations of heirs. I have not seen a period document describing any sort of formula or "rule of thumb" for determining ball size, but the information I have suggests the old hunters did not load 'em as tight as is recommended nowadays.
Notchy Bob