starting around the outside of the leaf.. maybe low K. pretty confident in that.
sounds like you are underfeeding, in general, if runoff is only 450ppm.
irrigation - water entire pot. if soilless get 10% runoff religiously. if soil, just enough to ensure you got the whoe volume wet. wait for top layer to dry. repeat. If not doing that, you are doing it wrong. Sounds like you are choosing a volume to give as opposed to what is required to get the whole thing wet each time.