Sweet skunk / Soviet grow

This is not a calcium deficiency. You give the right type and the right amount of nutrients. You also give the right pH. So what happened here is that the nutrients waste salts builded up inside your soil. To prevent this you need to give water till it runsoff from your pot. You want 10% from the given water to runoff every time you water. But for the first time I would give 2 liters with your nutrient solution and let it drain good to prevent it getting worse!

Maybe Calcium, but the leaves going crispy and stating green is reminiscent of root issues, is your medium draining well? is your PH on point in runoff? I'd check these few things first and adjust if they are off. Also calibrate your tools if you have not in a while. I myself do not like 20/4 lighting schedule and feel plants need a bit more down time. 18/6 is my ideal. Best Of Luck!

May be a Calcium Deficiency