Firstly, decide on mL (ethanol + water) to be used for your 80% ethanol solution and then calculate by how much you would have to dilute this solution to arrive at a 10% ethanol solution.
Might be useful here:
C1V1 = C2V2
V1 = Volume of stock solution needed to make the new solution
C1 = Concentration of stock solution
V2 = Final volume of new solution
C2 = Final concentration of new solution
Secondly, calculate the amount of drug required to be added as per the final volume of your 10% ethanol solution.