3 stone question

[sparkling]

If I want a three stone ring with rounds, should I use CT weight or millimeters across for determining proportion. I had heard that common proportions are 50% smaller than center, or sides 1/3 or 1/4 size of middle stone. If I have a 1.5 ct round, to have the stones be 1/4 size of the center, weight-wise they''d be .375 ct, but proportionately they''d be 18mm if my round is 74 mm.

Advice anyone? I want the side stones small but not too small so the center pops.

Thanks a bunch!
Sparkling