Have you googled this rust thing? I don't think it's mg def because that usually begins as golden yellow spotting in between the leaf veins. I'm wondering if you've given too much of a fert, but since it's been two weeks since the last dose, it's really hard to tell what's going on here. The damage appears random and brown, like heat damage from lights that are too close, but at 400w at 12-18 inches, I don't think that's it either. It's a tough one. I just did a web search and it looks like fungus can cause brown spotting with leaf curl, which is what you have going on there.