Coco is easy. I've been growing with it for years with great success. I don't use calmag either. The key is to use a nutrient that already has adequate amounts of calcium. What I use has calcium nitrate as the main nitrogen source. You'll find that many of the less expensive hydro nutrients do that. The designer nutrients don't have enough calcium in them on purpose so they can sell you another bottle of stuff. Another thing with coco is that less is better. The feed charts for many nutrients have people overfeeding. As odd as it sounds, overfeeding can cause nutrient deficiencies.
To grow successfully in coco all you need for nutrients is calcium nitrate, a micronutrient blend, and maybe some MKP depending on the P/K amounts in the micronutrient. Any decent micronutrient blend will already contain adequate amounts of magnesium and the calcium nitrate will supply all the necessary calcium. It doesn't make sense that a nutrient line supposedly developed for growing in coco would need to be supplemented with additional calcium or magnesium.