Light burn

Definitely not light burn at those levels, but i think your calculation of DLI is off. Tip burn can be deficiency or tox, so more charactersitics need to be considered. So, don't just assume one thing. need to eliminate possibiities. if this is the 250w AC infinity light. they are a bit light on diode count, which impacts longevity and efficacy, but the the umol/s are much higher than what you seem to be measuring in a smaller space. Their specs on ppfd are utter bullshit, good to ignore those. According to efficacy, which seems a bit high given how hard thery drive the iodes at 2x the samsung spec sheet, is 715umol/s -- so lets chop that back to something more realistic.. 2.5umol/j*260w/s = 650umol/s is much better 650umol/s covering 1 m^2 is 650PPFD and over 18 horus per day would provide 42.1 DLI, which definitely could burn the plant. I think your DLI calculations are wrong, here, because i doubt you are covering an entire m^2, and if light is closer to cover a smaller area, the more focused beam angle will result in a higher PPFD/DLI -- it's relative to m^2 and hours of use so you can compare apples to apples using DLI. Use that light at about 20-24" and it'll probably solve the problem, if actually caused by too much light, which i still do not beleive is the case. Your node spacing is fine. This is more likely nute-related. Whether it stays at teh tips or continues along the edge can help discern what it is... you may need to let this progress a little further to be more certain. P-tox and S-tox can caused burns at tips. K-def will spread to serrations as well as tip all along outer edge of leaf. If this has happened slowly, it's important to only make a small adjustment, and let it play out before re-assessing it. Small issues are fine.. you can be patient. unless you have more pertinent info than what is listed here, i'd see how it progresses and using that to further discern the actual cause. PPFD divide 650 by your area of coverate (technically dictated by height and beam angle, but tent walls help focus it.. figure DLI will be slightly lower than what is calculated but not by huge amounts)... if less than m^2 you have a higher ppfd than umol/s produced. PPFD is what you use when referencing DLI chart with hours per day of use. Above 40 is a grey area for atmospheric co2... properly boosted co2 shoudl allow upto 50-60 DLI. if you are at 600ppm and it's not allowing even 40DLI without issues, then that's more evidence the co2 isn't effective as-is.