Personally I think it's not necessary to do any measurements on ferts specially if you are working with RO. I mean the idea is not to spend the time measuring stuff to the minute details with uptake and whatnot. Every tank will be different, and in fact even one single tank uptake will be different from one week to the other simply because plant grow and consume more. Then you trim.... imagine the headache. The idea with front loading is that all this measurement shenanigans can be put to the trash by doing a simple 80% or even 100% WC if you can and then dose again to reach the desired levels. Also use the TDS meter once a month to check if it's stable or not. Although it doesn't provide the breakdown of what is in the TDS, watching your plants is enough IMO.But the math only tells you the input, not the uptake which varies with plant mass, plant health, type of plants, light intensity, CO2 and other variables. You have super high light, high CO2 injection rate, healthy and hungry stems, so the uptake is on fast lane. Math can only tell you so much. Ultimately, actual measurements count most. Unfortunately, there are no good home testing kits for K, bio available iron, and most micros, so mathematical guessing is the only means.