People often ask about appropriate sizes for gem designs – or whether a given design could or should be cut in a rough of a given size. There are a number of different ways to answer those questions, but getting it down to a formula could be a little complicated. I settled on a formula based on the average percentage of the sphere occupied by the average facet.
This table shows the relationship between gemstone size and facet count for RI = 1.54
Remember, these are broad and general guidelines, not absolute rules.