Novices and newbies discussion

I am currently trying to find the correct size back ring for, since i am planning to buy a made to measure metal chastity device. I am a little in doubt if have found the correct size backring, so i would like to ask here on this forum for advice. I have bought a rubber O-ring that have a inner diameter of 44 millimeter. When i am wearing this O-ring around my scrotum and penis, and i am completely flaccid, i can with a little difficulty push all of my index finger in under the O-ring. Would you say that i have found the correct size backring with 44mm, or should the back be a little smaller in size, so that i can only push my index finger under the ring, up to the big joint on my finger?