nutrient burn or light burn?

btw, you could start by trying a 18/6 without changing anything else... might be enough.. though i would wager an inch or two extra distance could be needed too -- if you don't want to dim some small percentage. in fact because there's already damage you may want to drop safely below "max" and work your way up after growth returns to normal. you can always use hours of use as a way to manipulate DLI just as easily as distance or dimming. each has pros and cons. with a photoperiod just safe to stay at 15+ hours per day during vege phase, then a max of 13h for bloom. it's not about hours, it is about DLI. hours is just 1 factor. more power at a higher distance could be potentially more wasteful with electricity in some small way, but would improve what light gets to the lower areas. more loss in absorption to walls with more reflections but provides wider angles and a more even blanket accross canopy. if only a few watts different it's pennies different per month in cost.

if it is too much light, it's barely so... 600 over 19 is just over 41 DLI. Environmental variables are different everywhere, so your "max" is going to be different than others. 35-40ish is typically a good place to start trial and error. When very close, it may take weeks to see a symptom. so, small and slow changes are best unless the damage warrants a bigger reaction. Some notes may help. It also looks like a nitrogen deficiency -- starting low on plant, moving up and from tip-in on leaves. clear sign, but could be related to light. I'd get your light in order, first, unless this continues to progress too fast, then i'd also bump N 10% or so in your formula. try to change the component of fertilizer that least impacts other nutrients the least while increasing N. are you using a photone app or similar? these are not accurate readings. it's just a lux meter applying some stock conversion factor without adjust from sprectrum flux density differences between your light and lux (that curve with spikes at red and blue that comes with light == SFD) so, those readings are useful, but they are not accurate. they will tell you proportional intensity. you can use this for evening out your light footprint over canopy - as best you can. the edges and corners will always be a little less when maximizing total area's light. is it 50% of center along edge or 66%? is there too much loss in center to get to 66? again, it takes time to fine-tunen these things. the math and phone app help you reduce the amount of trial and error required in a significant way if you use it correctly. they are tools and not an absolute answer.