Light burns from 50 watt Led panel

I think I'm experiencing light burns from a 50 watt led panel.

The plants where 3" from the panel and one of the plants has got burnt tips on upper leaves.

I've now moved the plants to about 7" from the panel

Does anyone have any experience with higher watt light panels and distance to plants?

ive never tried leds but i dont think they are hot enough to burn your leaves at 3" away. check the plant problem threads. either nute burn or nute def.

My 4 other plants are fine and i've been giving them the same nute solution as the one with the symptoms

The symptoms are exatly the same as the picture for heat stress in the plant hospital:

https://www.rollitup.org/marijuana-plant-problems/157345-have-plant-problem-check-here.html

Temp is 22C in the cabinet around top of the plants

edit: I have flushed the problem child, how long before I can expect to see any improvement?