I would start off by adding about 75 to 100 ppm of cal mag and see how they like it.
If you are using R/O water and your starting ppm is 20, bring it up to around 100 with the cal mag. Then just add your other nutes as usual.
For example, with my SFV in flower I usually bring my ppm up to 200 from a start of 20 ppm with R/O. They seem to love cal mag. Other strains dont need as much. You probably dont need as much if you are running tap water because tap water usually has some calcium and magnesium already.