2nd attempt at growing, need help please!!! Someone review plz

Hello I am a semi-first time grower with a custom built wooden microgrow cabinet with dimensions 26'h x 10'd x 19'w. I modeled it after an xl pc grow box case I found online, but had access to a laser cutter and auto cad so I built my own. It has one 120mm pc case fan for intake and two 120mm case fans stacked on top of one another for outtake blowing through a carbon filter. My first attempt almost made it through until last few weeks of flowering, but my 300w (150 true watt) led panel seemed to have caused bad light burn and my plants died off. I was growing in soil with fox farms nutes. I didn't have the best ph test kit and left it on for days without watering it so of course it died. I'm more serious now and am using the same grow box but this time made a small two net-potted dwc system. My seedlings have been sprouted in rock wool and I have a digital ph tester this time and will be more diligent on keeping proper ph levels. I'm using the flora grow trio series nutrients. Right now my seedlings are in the rockwool cubes and have just barely started to show roots so I planted them with hydroton. Everything seems to be ok so far but I am afraid that this LED light that I am using will kill my plant again? Last time I LST'ed the previous single plant and I was able to maintain a distance of about 5-6 inches from the led lamp. Before it died it strangely didn't start to show signs of light burn from the top of the plant, but more so the leaves at the bottom started to yellow and then brown and stuff and die, then all the leaves started to do that until the top started to curl up and shrivel and die, even though I tried watering it. I can't tell if it was the LED that killed it, the poor keeping of ph'ed water for soil, too many nutrients which I think I might have used... or what. My question is, is this led light too close to the plants? I bought a good brand so its not a cheap one. I'm going to try and scrog this one and keep my plants short. They are autoflower bubblicious feminized from nirvana. I wish I had regular fem seeds so I could control height better. I'll do that for the next one. I really only want to grow enough for personal medical use only. Should I buy a smaller wattage led grow light, possibly a grow bar light or small ufo or just use cfl's? Issue is is that I cannot buy a high wattage cfl with the hood on it because it would take up too much height. I could mount multiple smaller bulbs on a power strip but it doesn't have the penetration that LED has. Are there small led lights that would be ok to keep at a close distance, of about 5 inches or maybe 6? I appreciate any help and I'm sorry that this was so long.

 

Thanks for your response I appreciate it. I think that it is too close, damn it's a really good light too. I was hoping that maybe a much smaller LED light would be ok instead of, oh well. HID would be ideal but I don't think that I could use that because of heat issues correct? Like even a small one? I've never seen one in person so I'm not too familiar.