Here is the information from Lotioncrafter on it:
Germall® Plus (Powder)
A patented combination of 99% Diazolidinyl Urea (DU) and 1% Iodopropynyl Butylcarbamate (IPBC). Germall Plus is a highly effective, broad spectrum, non-paraben preservative for oil-in-water and water-in-oil emulsions and water soluble formulations. It has no known chemical inactivators and is compatible with virtually all cosmetic ingredients, including proteins and surfactants. Germall Plus is a white, fine, free-flowing hygroscopic powder. Not recommended for products intended to be aerosolized. Shipped with a dessicant packet to maintain product integrity.
Recommended Use Levels: 0.05% - 0.2%
How I would do it to take some of your water from your total measurement and then add the preservative to it and then back into your mix....