Generally, "seedlings" have enough nutrients in the seed itself, just enough to make it through the first little growth stage, which isn't all that long really. Probably what they were referring to? Once it has the first set of true leaves, it will start searching out for more nutrients and utilize them. In days though usually, def not 3 weeks, and especially in a hydro system of any kind..
Whats the original starting PPM of your source water? Using tap? What scale is your TDS meter? You probably don't need to be supplementing that much calmag, or even at all. More of a soiless\coco thing, and not so much with bare roots. I haven't bought or used any kind of specific calmag supplement products in over a decade of DWC growing anyway..
800 PPM is high IMO. That's like 80% strength with the nutes I use. I run smaller plants like that around 3-350 for a bit. Then I gradually bump up around 500 or so all through veg, no matter how long of a period. 800 is about the max I ever go with DWC, even in full flower mode with huge plants, with most strains. Maybe higher with a perfect dialed in environment, with c02 enrichment? They never ask for more IME, but many growers push it higher anyway and they still handle it fine. I tend to stay in the "sweet spot" range, where they drink exactly what you're giving them. As in, the water level drops, but the PPM stays about the same, and hopefully your PH gradually rises all at the same time. Thats what I go by, and not any feed charts for the most part..