Lyle,
I think you have your question backwards. Perlite does not absorb moisture - it is fired volcanic porous glass. By not adding in enough Perlite into the Potting Mix will cause the Mix to be too moist.
With the Sta-Green, which contains almost no Perlite, I would use 2 scoops Perlite to every 6 or 7 scoops Sta-Green. That will give you about a 22 to 25% final ratio of Perlite in the Mix.
Raybo
