Optimal pH for solution, when growing in 70% Promix BX, 30% perlite.

It's about the type of fertilizer you use, not the physical substrate that holds the water. The charge that comes with BX or HP bales is ~1EC. It's very low and it'll be exhausted the first week, so it really does need fertilizer from day one despite the charge of elements. I'd suggest shooting for 6. this gives a little leeway on each side if things go wrong. If you go a couple tenths lower or higher, nothing bad will happen and you stay near the acidic end of a good range which should improve results compared to more alkaline pH. If you treat it as a soil and use 'soil' fertilizers, 6.5-6.8 is what you want because it helps with availability of N, which is not necessary for ingredients that provide N in hydro/soilless fertilizers. The molecules that deliver N need microbes and other forces to break it down to be plant available. This difference is behind the 'why' even if i can't explain in detail. The ingredients for soilless fertilizers are 100% dissolved and 100% plant available upon going into solution.