If in coco, right away. I also rinse and buffer the coco prior to using it to ensure there is enough calcium and magnesium present to begin with.
If in soil with nutrients in it, I usually start CalMag around day 20-30 from sprout.
I saw that video you made with I forget his name regarding nutrients. I’m using fox farm ocean forest so you said about day 30 for first feed or so. The same applies to cal mag then.
Pretty much yup. You could actually do CalMag a week sooner and should be fine. It's rare for cannabis to become toxic in calcium and magnesium.