Rusty spread mid flower…

calcium and magnesium are two different things. I do not see a magnesium deficiency symptom. The spots could be related to a Ca deficiency. So, if you also increase Mg, this may be exacerbating the problem. Leaf symptoms are not discrete. Definitely have some dark green foliage with a lottle glossiness, which points to a N toxicity. Too much Mg or Ca can cause dark green foliage too. Heck, you could even be locking out K. N, Ca and Mg at high levels can fuck with K availability. So, if you choose a path, let's say dumping more Ca into it, and it gets worse, you should revert and consider another path. If systematic about it, you should figure it out over the course of a grow or 3 depending on luck with your initial guess. If you change soils or constitute it differently, you will have to go through that learning curve again. It's tough to say anything specific with soil. There are unknowns, like what is the medium providing and what are you supplementing? frequency of frtilization is only a problem if feeding at too high of a concentration relative to frequency. If this weren't true, then every single soilless grow would be a failure and hydroponics wouldn't work. No matter what method, they work best when it results in a proper ratio and concentration of nutes being plant-avaaible around the roots. If sticking to good watering practices, this shouldn't be a problem -- fully saturate, wait for dryback, repeat. Don't overthink it