+1 Bru'n Water
To reduce from 7.2 pH to 5.5 pH would take 0.09 ml/gal of 88% Lactic acid.
It's going to depend on the amount of alkalinity more than the starting pH. In BrunWater, I put in 215 ppm for alkalinity in the sparge water acidification tab (263*50/61 to convert from bicarbonate ppm, according to Palmer
). For a desired water pH of 5.5, it calculates 1.21 ml / gal of 88% lactic acid.
You can also do this in Kai's water calculator; just set the sparge water volume to the desired amount of water to treat and scroll down to the "sparge water acidification" section.