I think promix recommends 5.5-6.2 for ph. Id try around 5.8-6.0. Mine stsrted hsving problems at ph 6.5. So i eould think a bit lower than 6.2 would be best. All i can offer is an opinion. Trust your gut.other than starting nutrients early on low dose and ph for soilless, use cal mag starting sround week 3 especially with the qbs i dont see where youll not be happy. Im actuslly about to go get another 2 or 3 bags of promix. Worried they might not stock it over winter. I doubt they do and i might be screwed til march or april.