Nutrient deficiency which kind and/or light burn?

At first, I thought I was going to see a PH that was preventing the plant from absorbing calcium - but your PH is spot on... What I think you're dealing with here is too much nitrogen which can lockout calcium - and she's showing definite signs of a calcium deficiency... so your instinct to add calmag is a good one BUT you've got to cut back on that nitrogen! I use Fox Farm myself and have found GrowBig to be way too strong using it according to their schedule. A couple of things I do: first, I never use more than 1 tsp of Grow Big per gallon - ever. When I first introduce nutes to my autos, I only use 1/4 tsp to a gallon (about week 2 or 3).. the next week, I go to 1/2 tsp and then to 1 tsp which I maintain through pre-flower. When they go into flower, I start knocking GrowBig back... by week 6 (which is where you're at), I completely eliminate GrowBig and instead add Tiger Bloom which has a BIT of nitrogen in it - and it's all they seem to need. So that is what I would advise you to do - stop feeding the Grow Big! Add some Tiger Bloom which will give you more potassium and phosphorus... If you see the problem getting worse (the currently damaged leaves will not recover), then you're going to have to do a hard flush to get rid of the excess nitrogen in the soil (you're growing in 3 gallon pots so that means running 9 gallons of ph'd water through adding 1/2 strength nutes to the last gallon). As for the calmag - only 1 tsp per gallon and ONLY once a week or you'll end up with too much calcium and have another issue to deal with! Good luck!