you can use C1V1 = C1V2 formula to calculate. However, I am not sure this is the optimal way to do it as the ethanol will simply evaporate from the plate and you won't have the calculated percentage, therefore you are better off doing this in liquid. Also be careful of which ethanol you use, standard 70% ethanol that is used for disinfection contains additives and might interfere with your experiment, therefore, you should use analytical grade 95% ethanol as a starting stock.

Prepare Malt extract agar medium, autoclave and dispense in sterile capped test tubes which are preferably graduated. Say 40 ml of this medium aseptically dispensed in 50 ml capped tube, when medium temperature is about 65 C, add 10 ml ethanol, quickly mix by gentle vortexing and immediately prepare agar plates. Ethanol evaporates quickly at about 68-70 C. I trust this shall work to prepare agar medium with 20% ethanol. Same way to or high ethanol content agar medium plates can be made. Good luck!