Salty, it might be worth merging
these few topics into one, when you or other mod find time for it
(I believe there is even more of them, even if in all of them there are some formulas, none is 100% reliable).
Huy, I played quite a lot with that, and shared the complete collection in Swap Shop
here. Before that I looked everywhere for reliable formula that will work all the time, like other formulas do, but in this case there is none known. The best you can do is what you've been doing, put a combination that will give you the highest chance, and arm yourself with patience. If you're not playing a legacy or dynasty, than you can quit without save, which I did, place the combination, save, transform, if you don't get what you want, exit without save and repeat until you get it. I usually used 4 spire cuts /4 other gem in other cuts, preferably 3 and 1 (example: 3 emerald cuts and 1 pear cut Rainbow gem and 4 Tiberiums).
Good luck with the attempts and if you find always working formula be shore to share it (and for woohooium too, I still can't find a formula that work after LN).