Leaf septoria is highly doubtful - but you can read about that fungus here: https://www.ilovegrowingmarijuana.com/growing/marijuana-diseases-leaf-septoria/
I think you're right in your assessment of calcium deficiency - but it's only just starting and I don't see the need for a full flush... If this were my grow, I'd back off on the amount of your Rose & Flower nutes as it contains a lot of nitrogen which may be causing a lockout of the calcium... I'd also add some calmag to your nutes but do this only once a week. If the problem gets worse, then you may have to do a full flush but right now, the problem is only a minor one.
Nice job on the scrog and lollipopping, btw! Good luck....