Most nutrient manufacturers base their formulas on the assumption that you will probably be using normal tap water (EC 0.2 - 0.4) which already has some calcium and magnesium in it. If you use RO water you will have nothing in there to start with. Canna calmag agent can be used with very soft water like i have here OR with RO water. Just add enough to bring your water up to an EC of around 0.4. Then you add in your choice of nutrients till you reach your desired total EC and check/adjust ph if necessary.
I’m growing in coco so i never exceed EC 1.4 and that’s only for a short time (mid bloom). I let the ph drift between 5.5 and 6 to make all the nutrients available. Most mineral nutrients these days are chelated and should be available over a fairly wide ph but iv’e been caught out before so i still adjust the final ph.