dont worry about using 50 Litres on a 1st clean as that will get less with regular cleans and with practice, the thing is to always make a good impression on 1st cleans and win the customer over, that way they will come to trust you and recommend you to potential new customers.
remember also that conny roofs can aslo take a lot of water depending on their size and condition, this is always worth knowing before you quote them as they can also take much longer than you thought they would.