I have spotted what I think is your problem. First, definitely pay attention to the FoxFarms chart and stop guessing at 1tsp per half gallon. Use the appropriate nutes for the age of your plants in the correct amounts and definitely adjust the ph to 6.3-6.8 BEFORE you pour it into the soil.

But here is the reason for your rust spots I think. I am betting that you are root bound. That plant looks awfully big for that pot.