The first consideration is your source water. Look and see how much calcium is there. I have seen tap water with too much calcium right out of the faucet. the range is 80-140 mg/L
Then the type of media you are running:
Soil probably has some lime or dolomite lime in the formula - you will need no extra Cal-mag.
Coco is a needy substrate. Use nutrient lines for Coco and you should not add extra Cal-Mag. Using other nutrients your coco should have been washed and precharged then 50 PPM of Calcium, Magnesium and a bit of iron just for the coco every fertigation. Then feed according to the vendor instructions. If the vendor says to add it - it may mean they don't put it in the base nutes. Read the Guaranteed Analysis.
Hydro is formula based and the calcium should be in the range of 80-140 PPM/L
When a problem is serious a full water only flush is in order. This wreaks havoc on your root biome and is reserved for a last ditch effort. When you have a small nutrient correction to make it is better to do a "Nutrient Flush" slowly over several fertigations. This is much kinder to the root biome and the plant will respond better.