Some people say try a ratio of 5:1 and if no luck go to 3:1. That's 3 parts water to 1 part hypo. Personally don't think people charge extra for black spots but I may be wrong, they would just name the price upfront. Some people may say to tell the client before hand you can't guarantee getting rid of the black spots but you will do your upmost but that's up to you. Be careful if you are using it around lawn and plants.