I start my nutes from time I first feed/water my coco, (before planting), with 5 ml of Cal/Mag+ per gallon. I start the MC at the same time, but at only 1 gram per gallon. Some growers start with 2 grams per gallon, but I've found that using MC at that strength will twist leaves and stunt growth on some, (but not all), of my new sprouts. I usually up it to 2 grams anywhere from Day 10 - 14. With that said, I should add that I use un-buffered coco which will pull calcium that normally would be available for the plant. Here's what came up when I goggled coco coir and calcium ... Quality
coco coir has been treated to replace some of these cation sites with a
calcium (such as
calcium nitrate).
Calcium has a stronger bond to the cation sites than potassium or sodium because it has a double positive charge as opposed to the single positive charge of potassium or sodium.