ok this cal mag thing
withought sounding like a arrogant son of a bitch I have a bit of advice for you ive tested 8 different led systems over the last 2 years mainly 3 watt diodes so I have a idea how to use them and am starting to get them pretty dialed in now
so I feel I can say with confidence and experience that it wasnt a Magnesium deficency you experienced at least not through the light system not at that stage of the plants life its very hard to get Mg toxicity so it wouldn't have hurt but you made a misdiagnosis bruv and was more likely the soil or feed was a little hot or slight overwatering
the other bit of advice is the size of light for the grow area
Led are fantastic and you will really dig using them but you really must forget the hype from the company's that sell them
firstly your starting height sounds good and during the grow you should be able to bring that light to around 20" once they hit flower and maybe a little closer during the final flower stage's
to close and you will notice the resin production will slow Its not like hps the effect of the light being to close is gradual and hard to spot quickly so using the height recomendations I just gave you is a guide to help this grow go smoothly
however in my opinion the 330 watts you are using to cover 16 sq ft you may find a bit lacking, in fact you are only using 20.6 watts a foot, I would say the min would be 30 watts a foot as a guide only and you may find yourself disappointed with the quality of the result its great for early veg growth but will seriously let you down in flower with that low a umpol
hope this helps
CC
ok 20.625 watts per ft