Crown and Pavilion Angles
This is common sense; but common sense is not common. Many labs use grading systems with minimum & maximum crown and pavilion angles that penalizes cutters who produce BIC’s and FIC’s. The American Gem Society Lab was first to adopt this ‘inverse proportion’ approach. GIA is likely to follow soon.
If you know a diamonds proportions, you can use HCA to check if it can be ideal.
The crown (top) angle on the orange FIC profile is 10 degrees steeper but the pavilion (bottom) is 2 degree shallower than the blue BIC stone – the variations compensate for each other. The light rays travel on similar paths.