Hi stevecim,

I've spent more years than I care to mention working as a Goldsmith & Jeweller here in Australia, using the British system.
I'm retired from all of that now, but still play with the odd bit of CAD work for other Jewellers every now & then.
Unfortunately the way a finger size is taken can vary greatly, especially when converting between all the different systems.
When using an existing ring to get the finger size on a Mandrel, some will use "leading edge" & some will use the centreline of the band.
That's not including factors like people using cheaply made & inaccurate Mandel's to take those required measurements from!!

This all means you'll have a wider degree of variance between what you need, what you're getting & what will fit when it's all finished.

Personally, I now ask for them to measure the Mandrel at whatever finger size, using a set of Digital Verniers & then use only that measurement.
A millimeter measurement will give you a far better chance of getting you what you're after.
This works for me because I'm doing work for other Jewellers.
If you're a hobbyist then your going to have to rely on your customers going to a good Jeweller & getting their finger sized correctly...
...Good Luck as it could well be "hit & miss" deal..!
(I refer you back to what I previously wrote at the beginning of this post)

From my experience the RingCircle script appears to be reasonably close when I've needed to use the British system.
FWIW, the script was originally put together by Micheal with input from Jesse, who's a US based Jeweller.

HTH, Colin