Yellow spots on the leaves

probably a fertilizer that leaves a component out so you have to buy a supplement for no reason. Looks like what you thinkg it is... Ca, i'd guess. But, make sure pH is in an acceptable range and consider any recent behaviour... have you been dousing with nutrients or being conservative? etc etc... eliminate the potential false-positives and then think about adding a source of Calcium. Ca takes months to show up... if oyu see a Ca problem, it started from the beginning, most likely. future reference. in soil. you can amend it with instructed amounts of garden lime per gallon and that will be cheaper than cal-mag supplements, i bet... not useful to do later in a grow, as it takes a while for those pellets to be useful to the plant.

I noticed in your diary that you said you just bought a ph meter, correct? If this is your first time PHing the feeding/water to a steady 6.4, you may have given her a PH fluctuation, meaning her root zone got used to a certain PH and it suddenly changed. Ph fluctuation will look like a number of deficiencies and usually includes twisting leaves. Given the solid genetics, she should recover as long as youre consistent. Her stem also looks slightly thin to me. How close do you have your lights? They may be too far away. This is just my educated opinion given what Ive experienced and read from your journal. I may be wrong. Either way, youre doing great! Happy growing!