RR says 5.5-6.5 is good for fast colonization. Since there is little risk of contams with this method I would suggest staying in this range.
Test ph after mixing all ingredients with water after letting it sit for a few hours if you wanna have a real good idea of where it's at.
There might be room for a bit higher ph in a spawning scenario, but I am no expert.
Also, I'd like to emphasize that the point of this recipe is HIGH POTENCY.