HortFACT - Soilless Media in Horticulture
Promix is not a hydroponic medium, and I wouldn't be aiming for the more acidic numbers that were being thrown out here.
Treat it as if it were a soil mix.
Nothing to fear with a flush, it's just another step in growing. Salts and nutrients build-up, which will lower your ph over time. Be careful of keeping the soil too wet for too long. (don't water just before a flush, nor just after. Can cause root rot.)
I add my nutes into the final 1/2 gallon of flush water, then let it be for a few days till next watering.
HTH...