The combination of soft, squishy flesh, mixed with a flexible string is not a good method to use for obtaining one's correct base ring size. I sold two 3 inch diameter rings within a one week period recently. Both men used the string method, and both rings were the incorrect size.
Based on the most commonly ordered sizes when people buy a chastity device from me, the majority of men require a 1-3/4, 1-7/8 or 2 inch diameter ring, and only about 5% of the base rings I sell are wider or narrower than these sizes.
Your best bet is to use rubber test rings that are approximately the same thickness and shape as the ring on the device you will be purchasing.
I normally encourage people to wear a base ring for 24-48 hours when testing. If the rings remains comfortable then you have the correct size. If the ring becomes uncomfortable, then it's too narrow and the test is performed again with the next wider ring.
Mature Metal sell complete rings sets. If you feel you only need a few rings to test with, individual rings can be purchased through this vendor...
https://male-essentials.com/Rubber-Cock-Rings-RRAS.htm
I hope this helps!
Mark
http://www.malechastitynow.com/