I have no experience with malted millet, but in general there is a range we try to hit for all styles. The issue to overcome is that different grains have a different affect on the pH (highly malted grains will lower a mash pH - ie more acidic, so you would typically counter that acidity with the appropriate water additions. Sparge water should be treated so that it falls into the appropriate range as well.
If you download the free version of Brew'n Water there is a lot of information in there to read, though it's main purpose is to design water profiles and predict mash pH. It's friggin gold man.
One word of caution though....if you need to acidify sparge water or whatever, do it in very small increments and then test with a pH meter...it is SUPER easy to over do it with lactic acid, as I've found out first hand. Once you overshoot an acid addition you're in sticky territory trying to recover.
Cheers!