Magnesium is used throughout the grow cycle, so if you have an Mg deficiency at some point, your plants will need it throughout.
First, buy an EC/PPM meter. Second, use only the amount of Cal-Mag to stop deficiency from occurring. Only your plants can tell you what that amount is. Third, to prevent nutrient burn, reduce the amount of nutrients. If you can identify specific toxicity, focus on the bottle that contains that specific nutrient.