We validate colony forming units by plating serial 10-fold dilutions of our cultures. In order to save plates, you can do the dilutions in 96 well plates, and then plate 25 ul drops from each dilutions - multiple drops per agar plate. Find the drops that have about 50-150 colonies and count the colonies per drop. Multiply this by the dilution factor, and then by 40 to determine cfu/ml. You can confirm your dilutions by counting drops one dilution higher or lower and making sure there is about a 10 fold difference.