Wit's are traditionally mashed, AFAIK, in the normal (in terms of mash pH - one could hardly consider the use of bizarre devices like the Stuykmanden normal) way. The wort is then inoculated with lactobacillus. They get the pH down to the desired range. They are then killed off and the yeast then pitched for the alcoholic fermentation. Thus you don't add acid to the mash to get the pH you want for the beer. You add enough to get the pH to the point where the starches convert to sugar and the bugs are happy. They will set the pH where they want it. You can, of course, use any acid you want for this first step. But you must then provide for the souring of the wort. You probably will not want to go through the lactobacillus fermentation and will probably, as most seem to, brew a normal beer and add lactic acid to the product until the pH is what you want it or, better still, until it tastes the way you like it. Caution: be careful and experiment with additions. It takes some time for the lactic acid flavors to meld with those of the beer. You can, of course, add the acidf to the wort but this is clearly riskier than adding it to the beer. With experimentation you will find the right wort pH. Keep in mind that authentic wit yeast strains are given a sour wort to work with which wort contains less fermentable stuff than a wort that hasn't had some of that sugar converted to HLac.
I really don't have very good information about the unmalted grains but they tend to have pretty high DI mash pH. Given a more reasonable target pH of 5.45 you would need roughly 30 mL of 10% lactic acid or about 2 mL of 85%. So yes, 100 mL of the 10% is too much (would take pH to something like 5.1) and 100 mL of the 85% way too much.
