I've never used Cal/Mag and have used R/O and Lucas, MB, FNB, 8/16 for 10 years.My plants would be dead without calmag.
Try a test plant out...
Instead of Cal/Mag... use Epsom Salts. Somewhere between a half gram to a full gram per gallon should be sufficient for most "mag-hungry" strains.
I'm sure you'll be surprised at the results... especially considering you're in coco.
Stay Safe!