Brown spots

This is neither a calcium nor a magnesium deficiency - you're already feeding at the max suggested rate for the cal-mag you're using. In fact, if you already have hard water, too much calcium could block other nutrients, especially if the ph is too low. Unless you have been seeing signs specific to calcium or magnesium deficiency, cut the cal-mag by at least 50%. This particular plant is likely the most sensitive, so flush it to remove any build up of nutrients. Good luck

Yeah, I'm not convinced it's a manganese issue as that particular deficiency will start with yellowing leaves at the base of the leaf and spread outwards towards the tip, eventually developing those brown spots... and certainly you're keeping your pH at a level where this deficiency should not appear at all. And I'm not convinced either that it's a calcium deficiency as those spots are usually much smaller than what your plant is exhibiting. I'm inclined to think it MIGHT be a fungus of some type... septoria comes to mind but as septoria progresses, it will cause small bumps to grow in those dark spots and I don't see that happening yet. I would read up on the subject, though, as you know best how this has developed, how fast, etc. If it is indeed a fungus, you're going to want to act quickly to protect the rest of your grow. A decent article on it is here: https://www.ilovegrowingmarijuana.com/growing/marijuana-diseases-leaf-septoria/ If, after you read that article, you decide it is NOT a fungus or at least NOT septoria, then I would support m0use's answer to your question... and she makes some very good points on other issues she's seen in your grow. Good luck!

High Mr Dan The Weed Man, This is a Manganese dificiency. I can explain you how to solve this problem but it is easier sending you a link from the growweedeasy website. Because they wrote a really great article about how to solve this problem with pictures that make it really clear. Here is the link bro: https://www.growweedeasy.com/cannabis-plant-problems/manganese-deficiency Just follow the steps that they explain and goodluck with the last weeks of this grow bro! They look great!!! 😋