Should I do full flush til runoff is 300ppm?

I'd give this more time to be certain of what it is. Fertilizer doens't workt he way you perceive it. You don't give it something one day and see results in 24 hours. It won't work like that often. you are using "flowering" nutes? These often skimp on N for popular belief, but the plant still needs most of the N you provided in vege, too. Although this looks like it is interveinal chlorosis, which isn't typically how the N-def progresses. Too much K is a possibility too. They often amp that up WAY too much due to popular beliefs in flower nutes. That can result in some chlorosis to lower leaves as welll as pale new growth -- if no pale new growth can eliminate this one. Could be teh start of Mg-deficiency, but you'd see spots of damage to go along with it. Also, it takes 30-35 days for the symptoms of an Mg deficiency to show up, so you need to look back a month or more to help diagnose that with any confidence. If one of the nute-elemetns is being fed too high, it could be locking something else out. Mulder's Chart would help deduce that. 1450ppm isn't high for soil, but it is for soilless context. so there's good chance something is too high as you are in coco. (this assumes .5 scale for converting EC to PPM, tds pens can have 3-4 different conversion factors, so the ppm reading is not an accurate number, so it's a bit tough to use as more than a ballpark figure). EC between 1.3-1.5 is a safe range for soilless/hydro fertilizer. There are a few trace elements that start at teh bottom (mobile), but these are highly unlikely to be a problem. Because you are soilless, this makes it fairly easy. Compare what you feed to the values below 120-130 (110ish in flower) N 40-60 P 180-200 K 100+ Ca 75ish Mg 100+ S This is a low EC formula but the ratios work well. Depending on your VPD you may need to increase overall concentration. Depending on local tap water, you may need more or less of things like Ca, Mg or S. but, if anything is drastically higher, it's a good suspect for nute-lockout. Refer to Mulder's chart to futher investigate that possibility.

I would recommend lowering your ec with small plants in soil your 1450ppm might be too much. From the leafs it looks like the plat can’t absorb enough cal mag. Wouldn’t go with a full flush, next few feedings give them a lower ec nutrient solution with some extra cal mag. This should hopefully solve the issue