Here's a stone that I'm looking at:

GIA cert
round brilliant cut
E color
VS2 clarity (light feathers on the peripheral)
1.58ct
56% table
61.8% depth
35.8 degrees crown angle
40.4 degrees pavillon angle
very good symmetry and polish

I used the HCA and got 0.9. The ratings are ex-ex-ex-vg.

However, I tried to play with the angles a little and found that if I use a pavillon angle of 40.8 degrees, the total score becomes 3.9 and ratings are gd-gd-gd-vg. Can you help me understand why such a small change in the pavillon angle (0.4 degrees) would change so much? Further, I understand that the Sarin report gives only averages, what should I do to ensure that the stone behaves more like 0.9 stone and not a 3.9 stone?

Thanks for your help!

Garry is traveling now and I'm not sure that he will be able to answer your question soon.

yes the pavilion causes 5 times the effect of crown angle or table size.

even if that stone is off symmetry it will still be very good, but hard to explain with time i have now.