Is This Start Of Nute Burn Or Lights Too Close?

Adjuster Wing Reflector on most narrow setting
400 watt MH bulb
27 inches away
Hydro DWC
2.5 Weeks Into Veg
300ppm - GH Nutes
Ph: 5.9-6.0

I want to know can the leaf temp be fine but the lights are still to close because I measure the temp with a infrared temp gun and it is always between 71-77 degrees. The leaf edges and tips are white/yellow so it's either nute burn but the ppm drops from 300 down to 225 in 3 days so can't be overfeeding right?

If you zoom in and look closely you can see the white/yellow on tips and edges of leaves

actually 300ppm is a little low in my opinion. I'd probably double it to 600 and see what happens. it's not nute burn. you could probably lower that light if you wanted too. 400 watts isn't much. but just make one chance at a time and see how things go. I'd increase the ferts first. maybe someone else will have an opinion.

Looks healthy to me buddy