With cosmetic [hair] peroxide 20 volume, what would the ratio per ounce of peroxide to water to reduce the effective peroxide volume?
As librarians we help point people to resources where they are can find the answers to their research, how to use teh resources, how to evlaluate, and so on. We rely on resources like articles, books and even web sites. In this case I am not finding anywhere where you can get that specific information.
If you are one of our cosmetology students it would be best to consult one of your instructors. If you are not a student I would suggest a professional cosmetologist or perhaps even a mathematician so that you can formulate an equation that would give you the answer.