I use pearlite, and the way I determine how much water use is by the squeeze test. I get the mixture completely saturated, then squeeze as much of the water out by hand as I can. When no more drips out, the mass is still damp, and I use that as the basis for my eggs. I then put the whole thing in the incubator, cover the eggs with the aforementioned paper towel, and check on them every other day or so to make sure the hydration is adequate.
Good luck!
Darin