Have you looked at brun water
? Select the water profile you want to brew and in your case set the RO water to 100%. From there you can add whatever you need to get to the profile you want. It is very in depth and the size of the spreadsheet can be intimidating. The simplest way since you are not going to have to enter your existing water profile is to go directly to the water adjustment tab. Just about everything you ever want to know about brewing water is there.