After precharging with calmag, I don't add more into the nutes until around week 5. And, then, I will use 2-3 ml/gal (I use Calmag Plus from Botanicare) unless I see the telltale spotting start on the leaves. I've had autos that didn't show a calmag def until week 8, and it just took a full 5ml/gal to stop it.
In my environment, I use well water, and it has 100-150 ppm of "minerals" in it. Most of that is calcium, so, as long as the plants are in a good pH range, they'll get some calcium and magnesium (and iron, boron, zinc, etc..) from the well water.
If using distilled or RO water, there's no "extra" in the water, so adding calmag is more of a concern

Thinking about it now, I haven't had much in the way of calmag issues since I started using coco drip setups. But then, I started using AN nutes with the drip systems, and I think there are a lot of little "extras" in their various mixes that might be compensating for the calmag addition.
Its great that you're asking these questions ahead of time

I did the same, and have to thank AFN members truu, and JAYAR and a bunch of others for giving me the good word. The first run under my DIY drip, I was expecting nightmares! But, it was an autopilot grow from the very start, and it went better than I could have hoped!
Let me know when you've got a journal up. I'd love to follow along for the maiden voyage!


ps: Canna coco is also a great product, if your hydro store only stocks one bagged form. Mine only has Roots now, unless you want coco bricks from Dog knows where... lolol