You can have T5 very close as it isn't as intense as other light sources. As long as the plants don't ever touch the bulbs, you can literally keep the lights within 1"-2" of the highest tops. The one caveat to this is that it is wise to have a small fan to blow across the bottom of the lights and top of the plants. This keeps hot air spots from building up in the canopy. It doesn't need to be a lot, a small fan to blow between the plants and the lights just to move the air will work fine.
On the Necrosis: What I had seen on the older pic was a light green margin around the leaves, and many leaves curling up like a taco shell. Those 2 symptoms are very classic magnesium deficiency symptoms. However, the necrotic spots along the margins looks like a potassium deficiency. These 2 can be confused when the magnesium deficiency gets worse. I am also seeing in the latest pics, leaf tip necrosis (or tip burn). This is often a sign of overnute (or Nuteburn). However, Nuteburn is often accompanied by leaves getting darker green, and the tips of the fronds curling under like a ram's horn. Also, in the pics, while I see some leaves with tip burn, I see many leaves without the tip burn. Most often, Nuteburn will show on nearly all of the leaves pretty quick.
Now you said above that it got worse after you tried to correct it as a nitrogen deficiency, but in the pm to me you said it got worse after adding calmag. Did you add calmag? what kind? did you add something with high nitrogen? When did you do this? Have you ever checked the pH of your soil?