If you don’t have a plastic Vernier Caliper then you need to get one. As I see it your going to have to maintain the correct length to width ratio of the stone yourself when cutting the girdle. You can get them on eBay for a couple of dollars.
This is the way the stone looks to me from the computer screen. G1, G6, G7, G3 will be using the same height stop setting, G2, G4, G5 another. So there are two height stop settings on the girdle producing a stone with a L/W ratio of 1.248. The index settings should produce facets of the correct sizes.
Hope that helps.
Frank.