The Refractive Index is “nice to know stuff” when it comes to searching the various databases of designs. However, the Critical Angle (CA) of your material is really what matters. Always cut the pavilion mains above the CA.
Here’s a handy-dandy chart that shows the RI & CA for the most common gemstones, along with the “suggestions” for the mains from three different reference books…
Gemstone Faceting Angles
Be sure to read the notes below the chart!!!
As far as how to change GCS, it’s pretty straight-forward…
Take an SRB for example with P1 = 42.5 & P2 = 41.5 (one degree less). If you want to increase the critical angle after examining the above chart, just click on the P1 tier and “Edit Tier”… That will allow you to change the angle and depth using the sliders on the right side of the application. Then do the same for P2, just making its angle “one degree less”, etc. Same goes for the crown mains, etc.
There is actually a better way to make ALL of the tangent ratio changes at once in GCS, but it involves using the “Manual Optimizer” after setting the RI in the “Render” panel. Study the tutorial PDF for GCS and practice a bit… it’s great.