How long ago was it up-potted? FFOF takes, at least, a couple weeks before it should have any nutes added, but I wasn't sure of the timing based on your description.
Do you feed every watering, every other, some other schedule?
Do you know the PPM of your runoff?
Good that you're adding calmag since you're using distilled. Is your tap water not suitable? It would be better if it doesn't have any significant issues. Distilled lacks a lot of micronutrients that aren't part of normal feedings (calmag is just one piece of that puzzle). Spring water, I've read, would be better than distilled, if you can't use tap.
It looks like the spots are only on old growth, which is good, especially if it isn't spreading/growing.
They do look quite basil-like, a little down-turned at the tips, and quite shiny, which points back at feeding too early with FFOF, but not sure. Some strains, I've had in FFOF, will do that just from the nutes already in FFOF itself.
Sorry, not really an answer, and instead a bunch of questions... but I'm leaning toward slightly over-fed...