Am I correct in diagnosing this as Calcium deficie...

Hi @NullHypothesis! You are right, it looks like a Calcium deficiency. The weird thing is that this deficiency only shows up onto new leaves, whereas you seem to be having issues with old leaves. Now the big question is: what could have led to this situation? Have you been giving some extra cal+mag during the growth? Your pH looks fine, are you using RO water? Could it be a potassium abuse, making the rootzone unable to handle calcium intakes anymore? You can add some cal-mag or epsom-salt into your next feedings, it's probably a bit late because the deficiency has been there for a while, but it worth a try, Happy growing 👊