Ok so here was my situation, we will see if it helps....
Im at about 6 weeks since seedling. My leaves were doing the exact same thing as yours, from the bottom up being worst to just showing signs of spotting. I still think i was coming up short on mag so I did what you did and bought some calmag. On a side note I was feeding at about 560-630ppm. I noticed the growth had stopped 3days ago so I whipped up a batch of 770-840ppm food because when i got home the yellowing and brown got worse, purple stems and on the bottom leaves the main veins were yellow. They also started drooping. I fed more food with calmag (I believe 250-300ppm) and whipped up some foliar spray for the effected leaves.
I woke this morning to see my leaves had boners and newgrowth was coming along. Some of the stems were equalizing still purple but you could see the transfomation back to green.
I think I was under feeding when I thought I was giving them too much. Maybe that was my problem and yours too. I think the plant was just eating itself to stay alive. Time will tell though and I will continue to post here til we get this thing worked out.