Joey, the concentration in units/microliter should be provided in the tubes for these reagents. Just divide the number of units you need by the concentration of MMLV-RT or RNasin. For instance, if you need 25 units of RNasin, and the concentration in the Promega RNasin tube is say, 10 units/microliter, then divide 25 by 10, and you'll need 2.5 microliters.