I don't think Sub ever endorsed the Myco grow - I did
I've heard conflicting stories regarding PM and humidity. I run dry - temps around 80 and humidity around 30-40, sometimes less. There are several strains of fungus that are normally lumped into the category of 'powdery mildew'. Some of them probably do better in different environments, and at least one of them does better or worse in certain conditions depending on what point it's at in it's life cycle(i.e. it germinates better in high humidity but grows better in low humidity). So dealing with PM isn't as simple as 'do this' or 'do that'.
Air flow really seemed to help, along with keeping my humidity up(for the PM strain that I had). But I'm in the southwest and those two goals are usually at odds. The more outside air I bring in, the more my humidity drops.
As far as the mycogrow, I think you can't really add it until you have roots - some of the organisms require carbs from the plant roots, so adding the myco's to the SS mix may just result in a bunch of dead myco's. I usually mix up 1/4 tsp in a gallon of aerated water and water with it a few times early on. When I noticed the anti-PM effect, I had added the solution to mature, flowering plants though.
Sorry - this isn't a PM thread, so I guess I'm threadjackin.