Not a direct answer to the question, but probably helpful nonetheless. I used the following table from Ultrahuman (see link below) to determine my ring size by wrapping a strip of paper around my finger, marking the circumference with a line and following the measured value in mm in the 'Circumference' column to the corresponding value in the 'Ultrahuman Ring AIR Sizes' column. My ring size 12 was spot on.

https://www.google.ch/url

If the link does not work: Google search 'ultrahuman ring sizing', images, ring sizing chart. I also tried the Ultrahuman app to determine the ring size, but that didn't work.