Thanks guys. The plants are literally eating themselves now, so a lot of the larger fan leaves have turned yellow, then crispy, then fallen off; especially on the Sour D's.
In my experience that's the sign of smooth-burning, sweet-tasting, properly flushed buds. I try to reduce the amount of nitrates in my smoke as much as possible.
As far as powdery mildew is concerned, I think they've been taken care of. The diluted skimmed milk spray helped, as did reducing humidity (between 25-30% now!).
I also noticed that the sour D's preferred the hotter, drier climate in the grow room and increased trichome production.
My growroom is a Homebox L, so just under 11 square feet.
For exhaust I have a 4" inline fan with a carbon filter. No intake fans.
For circulation I have a Honeywell 3-speed fan, and also my dehumidifier has a built in fan so constant circulation isn't a problem.
Humidity's really the one to watch out for; especially in summer (here in Switzerland at least!).