I don't know precisely when shot sizes were standarized with the choice being that of Tateham & Bros shot tower of New York. On this chart #10 (Drop) shot ran 848/oz. From a shot tower chart in an 1892 Lefever Arms co catalog depending upon the tower #10 could run from 822/oz to 1130/oz. There were 9 towers listed with #10 running between 822-874 for 6 of them 950 for 1, 981 for 1 & the 1130 for "Merchants Shot Tower Co, NY". Of the 9 towers 4 were from NY, 1 PA, 1 MO, 1 IO, 1 CA & 1 IL.
For the Tateham system whole sizes vary by .010" With #B being .170". Taking 17 & subtracting size as decimal will give the shot dia , thus .17 - .06 = .110" dia for #6 shot, .17- .075 gives .095 for #7½ & .17 - .10 = .07" for #10 etc. This also works going up except you have to remember the exact order of the steps. Above b there is BB, BBB, T, TT, F, FF & then #4 Buck. Above #4 buck they no longer follow these .010" steps.
