I have black spots and burnt edges on a couple lea...

Some of the tips in the plant have a downward point to them, that is reminiscent of to much nitrogen. But only some of the new growth have it so that's a good thing. To much Nitrogen can lock out calcium. The spots do look like a calcium and magnesium deficiency, so it could be to it need more or its locked out. The ratio or concentration of that brand of calmg is 3.2%Ca to 1.2%Mg, ruffly 3:1, ideal is 2:1 its feeding recommendations are 2.5-5ml but can not find out if that's per L or Gal to compare it your grow. Have you monitored the run off PPM? Maybe all the right amount of nutrients are going in but there is to much salt buildup cause lock out. If your PPM is 100+ higher then what it is going in you need to do a flush. Good Luck and Happy Growing.