Light Burn, Nutes, Or just normal?? Input please

Light: 600w HPS air cooled hood
Nutes: GH Lucas formula (ppm in reservoirs range from 900-975ish)
Ph: been pretty stable about 5.9-6.2
Going on 4 weeks flowering
Overall plants are healthy, I just did a midway flower res change just to be safe since it is DWC. Using Lucas Formula..addback.. My ppms have been shifting slightly, even after just adding plain ph'd water for a day or 2. But I am leaning towards light burn over nute problems. But the white sensor in the pic always reads between 74-78 degrees but my leaves do look a little burned as well as some feel kinda dry. You can see the slight yellowing/ some brown edges Any input would be great. Buds are growing everyday and temps should be ok so this is baffling me

One more thing, due to some unintended stretching, I have had height issues, gradually tying down some of the many colas (several cant be seen in the shot). The bottom of the hood is about 10-12 inches above center plants.

After some reading, I have reason to believe it could be a Cal-Mag Def. Anyone familiar with this def beyond copy and pasting definitions?

I know you say that your temps are 74-78, but it is showing clear signs of heat stress. See how the end of the leaf cuts are browning. Either your light is too close or your temps are wrong. Try backing your light off them a bit. You definitely don't have an over nute problem because they seem to be vigorous. Try the light I am sure you will see them green up again. If you cut nutes out completely when they need it, you will start showing signs of other def.

yeah thanks they are in a grow tent and i ran out of height but i macguyvered it. I just folded the top cover of the tent back and hung the light from the ceiling above it and just covered the open areas with panda film at the top. Temps are good now and the fan leaves seem to be relaxing. appreciate it

is not slight browning and yellowing of fan leaves part and parel of budding process? i am 1 week behind u but did grow years ago - budds looking good

While I can definitely see some signs of heat stress with the leaf edges -- I think your nutes are going fine. Generally the flowering process does this anyways, and it is in part due to the fact your nutrient formula changes to having less Nitrogen and more Phosphorus and Potassium. Lower levels of Nitrogen ill cause the plant to rob it from the leaves when needed if there is not enough in the 'diet'. This is generally a desired effect specifcially prior harvesting.