leaf tips turning light green

I have this problem and read that this is light burn. Doesnt seem like it should be, because my light is at 50% and plants are 28" away. I have a FC-6500 light. According to chart the plants are getting around 500-700 ppfd. Any thoughts? The two plants that show this are autos.

Looks like tips got nutrient burn. Mine get like this in mid flower when im juicing them with PK. I usually just water without ferts next time and rotate feedings