The rule of thumb is for each 45.5kg (100lb) of grain, 1% or 455g (1lb) Acid malt will drop mash pH 0.1. The actual amount of lactic acid present on 455g of Acid malt would be 455g x 2% = 9.1g.
To use a 88% solution for a similar quantity of lactic acid, you would need (9.1g/.88) 10.34g of solution. At a density of 1.21g/cc, 10.34g/1.21, amounts to 8.54ml/45.5kg
For beers without roasted malts, the HomeBrewTalk "water primer" recommends your grist include 2% Acid malt.
For 45.5kg of grain, 2*8.54ml = 17.08ml lactic acid solution or 0.38ml/kg. At the recommended dosage, a typical 8.5kg grist would include 170g of Acid malt or about 3.25ml of lactic acid solution.
Using the noted calculations, I mashed 8kg of Weyermann Pale Ale Malt and 0.5kg of Domestic Flaked Barley with 3.25ml of lactic acid. The water:grist ratio was 1.3qt/lb. Important note, my baseline city water is extremely soft. Also, the CaC03 alkalinity is below 35ppm (~28). It's great water to build on. I treated the entire volume of hot liquor (50l) with 12 grams Gypsum and 8.4 grams of Calcium Chloride, the resulting mash pH was 5.6 @ room temperature. An additional 2ml of lactic acid, total 5.25ml in the mash, resulted in the idea room temperature pH of 5.4. I actually took a reading at mash temperature as well, hard on the meter I am told, the pH read 5.1-5.2.