I would skip the acid malt and just add straight lactic acid. Your LHBS should have it. Don't ask me how much, you'll have to play around with it on your own, although it's all guesswork anyway, since you don't know exactly what the Brett will do with the acids and with the rest of the beer. You can use any leftover lactic acid to adjust mash ph in future brews.
Say no to intolerance: love gluten and lactose.