For Hoppybrewah: If your water has an alkalinity of 218 ppm as CaCO3 (measured to end point 4.3) at pH 9 then it should take 3.1 mL of 88% lactic acid to move 10 gallons of it from pH 9 to pH 7. To take it from pH 9 to 7.1 requires 2.7 mL and to take it to pH 6 requires 9.9 mL and so on. Yes, indeed it does depend on the pH you are trying to reach. It also depends on the pH you start at. If your water had alkalinity of 218 ppm at pH 8 it would take 9.6 mL of 88% lactic to move it to pH 6.
For natmartin/DannyW: We know enough about 5 Star 5.2 to be able to draw some conclusions. It is "a proprietary mixture of phosphate buffers". As such and given that it is supposed to buffer to pH 5.2 we can determine the relative amounts of monobasic and dibasic phosphate salts in it but not, without analysis, whether they are sodium or potassium or some of each. Given the recommended dose the sodium and or potassium loads, however you split them, are more (something like 150 mg/L sodium or 230 mg/L potassium or half that much of each or some other combination) than I want in my lagers not to mention the fact that the buffering of a phosphate system at pH 5.2 is pretty poor. A citrate buffer should perform better (nearly 10 times the buffering capacity at 5.2) but might well taste too citrusy. Investigation up to this point has been at the computer and in the lab - not the mash tun i.e. we're still looking in to this.
