I don't use a formula. I mix it by hand and by feel. When it clumps just right, I know that I've got it. It is something that I would have to be there to explain right though, or maybe videotape, but I don't have a good filming camera. I mix together perlite and vermiculite. I like the big chunky verm, not the thin powdery stuff; it seems to keep form better and it just feels right to use it. Bigger balls of perlite also; not the teeny crushed stuff.