+1. Mash pH is important, not the pH of the source water.Since the pH of blended water is pretty much useless info for brewingPardon my obvious stupidity but why is that a priori true?
Water, regardless of mineralization (within reason), has essentially no pH buffering capacity. The variables that drive mash pH are the water's residual alkalinity and the inherent acidity of the grist. Two water supplies with vastly different pH, used to mash the same grist, could come out to exactly the same mash pH.