Do all your fert mixing BEFORE you even touch the pH. When it sits, CaCO3 in the water, and other carbonate from atmospheric CO2, can react with the acid and neutralize it.

I recommend just adding your PBP to your tap water BEFORE you adjust it. It's buffered. You may find that you mix up a gallon of your nutes, shake it around, test it, and find that the pH is already correct. If it isn't, it will take less adjustment to get it right.

Oh- and your meter is a perfectly fine one.