What are your base nutrients bigeasy? I had a problem with rust spots on my Skunks last year and I was never able to correct it, showed up about halfway through flower and just got worse, harvested good though. Had some NL's in the same room that never showed the problem. I'm trying to figure out this yellowing on my lemon kush right now.
You say you're using a tsp per 2L of water? What kind of water are you starting with? You could be looking out other nutrients if you're using that much cal mag