The 400 ppm taste threshold for lactic in beer was presented in Malting and Brewing Science. The important things to remember is that this is the median response in humans, some may detect it at lower concentrations. In addition, malt and yeast also contribute lactic acid to the beer. Malting and Brewing Science indicates somewhere in the range of 200 to 300 ppm is contributed by those sources. Therefore you can't just add the equivalent of 400 ppm lactic acid to a beer or the water. The allowable amount will be less. I've suggested that limiting the lactic acid addition to produce 200 ppm or less concentration is wise.
Since lactic acid is a monovalent acid, for every 1 ppm of bicarbonate you neutralize with that acid, 1 ppm of lactate is added to the wort or water. So using the Bru'n Water calculator, you can quickly see what you are adding to the mash water since the acid addition shows up as a negative Bicarbonate addition. Keep that value below -200 ppm and you should be good. It turns out that for 88% lactic acid, that equates to about 1 to 1.1 mL acid per gallon of water. So 5 mL in 5 gal is safe. 10 mL is probably pushing it.
PS: Lactic flavor can be a pleasant component in some beer styles, so don't fret if you exceed this limit. For instance, doubling that safe 1 to 1.1 mL per gallon dose should produce a notable lactic taste. Good for Wits, Berliners, etc.