Lighter colored leaves at flowering sites

Lack of nutrients can cause yellow leaves like yours. Ph is the last thing I assume unless it’s growing in hydro

you're at the early flower stage, did you drop the nitrogen back to far to early?

400w in a 3x3 space, give or take? Was probably the light. As far as guesstimating proper power, totally depends on how accurate that spec sheet is, otherwise it's 100% you fucking around with height and power until it the plant grows properly without symptoms. I'll assume you can choose the hanging distances based on light distribution? with a light like that, probably 20-24" is a good idea. QB, cob led and those older smaller framed lights need more distance. For 18h, you only need 56 umol/s PAR per sq ft. This is where the spec sheet comes in. Multiply your total umol/s PAR needs. 56 x sq footage of canopy. Take that total and divide by max umol/s PAR output of the light. That is your starting power percentage. Minor adjusments will always be needed, because "max" DLI is a moving target unless temp, rh, and ambient co2 are all tightly controlled. Regardless, it's a good start with minimal adjustment needed, assuming the spec sheet was accurate. Read up on Daily Light Integral and PPFD, if you want to understand how to properly measure and compare light provided to a plant. Should be easy to extrapolate the why of everything said above. Learn to fish.