Nute burn or light burn ?? Or somthing else

possibly potassium related. Could be too much light if that damage is predominately at the top, because 45cm is very close for a typical grow light at 100% power. if in soilless simply calculate ppm of K and if drastically lower than 180ish ppm, it's probably low in K. you only need about 32-40w per sq ft fro 12/12 light schedule. The efficacy of the light will be the determining factor of that range of watts. If you are on an 18/6 wiht autos, you need 2/3rds of that watt range. 21-28w give or take, i'm not doing ht emath in my head right now. So, if giving a lot more than that for either light schedules, it's definitely too much light. umol/s per sq ft.. can use the spec sheet to guesstimate this stuff accurately too. divide the umol/s PAR your light produces by area and anything over 80-85umol/sq ft for 12 hour operation is probably too much. Again, take 2/3rds of that for 18/6. These values are proportion to hours of operation. It's very easy to know if you are WAY over... tougher when closer to that max threshold... have to observe how the plant grows to determine that.. if nodes too tight, it's too much. too lanky, it's too little. When you find and optimal level in vege, it's simple algebra to adjust it for 12/12. You've already done the hard work now it's just multiplying the power% by inverse of change in hours of operation. (Less hours/day needs more power per hour, obviously)