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.