Plug 4 numbers into the HCA
Depth %
Table %
Crown angle in degrees
Pavilion angle in degrees.
(I've read that cCrown and pavilion heights in % are less accurate.)
Reject round diamonds that score over 2.0
HCA uses averages of the 8 crown and pavilion angles around the diamond so HCA is a rejection tool, not a final acceptance one.
The angles may vary wildly (bad) but average out to a good number.
Get an Idealsope image on those scoring under 2.0.
Unlike the HCA, an Idealscope image will reveal if individual crown and/or pavilion angles vary.