Hi Jeff,
I have found the fuzzy and contradictory conventions of ring sizing
very confusing. I now only use the ISO system which is simply the
internal circumference of the ring in millimetres. The crucial thing
for me is that I have a set of 36 ISO rings a calibrated mandrel. If
I am making a ring to size I measure the client’s finger and use the
matching mandrel to make the ring.
I do have a conversion chart to give the equivalents in the US
number sizes and the British letter ones but as you can see the ISO
system scale is a great deal more even and precise