I may well be wrong here but did not know there was a "Proof Tolerance" Per Se. I was of the opinion the proof house simply measured the bore, stamped its gauge according to that size & proofed it by the charges for "That" gauge. The chamber marking then tells you what shells to put in it.
If you look at the proof charts a grouping of in-between gauges both larger & smaller will normally have identical proof charges.
.729"-.739" will be stamped 12; .740"-.750" 12/1; .751"-.762" 11; .763"-.774" 11/1 & finally .775 as 10.
Th actual proof charge will change a bit at the larger end, but any of these size bores "Could" carry a 12ga chamber.
