Well, from memory, a 12 point round will be a bit over 3 mm's (around 3.15 for full-cut round) in diameter if that helps. You'd have to divide by the projected bracelet length in MM and allow for various setting tolerances as required for the style of the jewelry. Good luck!
I think it depends upon the setting too and how close the diamonds are together. Four prong and half bezels seem to have the stones closer together than in a three prong style.