With <10 PPM RO water I always start building my feed with 5 mL per gallon of calimagic. I like the calimagic because it has less nitrogen than the other cal mag products I have used. Thats just to replace the minerals that should be in the water. When I switched to using my tap water (180 PPM) I learned that I need just a hair over 2mL per gallon of the calimagic to prevent calcium deficiencies and I use a third of a gram of epsom salt per gallon for the magnesium. This has worked very well with my tap water and GH flora three part. So I look at the calcium / magnesium as just getting the water right. Then I start adding my base nutrients, they also bring some added calcium and magnesium so I need to keep that in mind when determining how much calcium and magnesium we are adding.