To add just a little to Gr8day's formula, I have posted this on several occasions incidently, if you want all sizes from 1 ga down to 172.28ga (.300cal) to round to within ±.001" of the dia' s given in British Rules of Proof it is necessary to go to 6 decimal places for the 1ga. There is a very small range which will give correct figures but about the middle of the road is 1.669285.
Also an interesting aside is if the wt of lead as given in various publications (.4096lb/CuIn) for a long number of years is calculated there will be found a slight (very slight) error all the way down the Proof House chart. A few years back we discussed here the possible cause of this. The most plausible to me was the possibility that the ball used for the calculations had been rolled to insure roundness & in the process was in fact slightly compressed. I cannot now recall whose idea this was to give credit. The error only amounts to a very few thousandths of an inch & would show the sizes to be slightly larger than the charts. Plugging in the above listed wt of lead gives the 1ga a dia of 1.6706".
