Runoff ppm's/ EC

I’m trying to determine how much nutrients my plant is uptaking with each feed. let say today I feed my plant 0.55EC and runoff comes back at 1.43EC. My previous watering EC runoff was 1.9. Does that mean my plant absorbed 0.47 EC worth of nutes or do I need to subtract the 0.55EC from the 1.43 and then subtract the result from 1.9?

math example:

1.9(previous ec runoff) -1.43(today’s EC runoff)= 0.47 (nutes absorbed)


1.43(today’s EC runoff) -0.55( today’s Nute input)= 0.88
1.9(previous feed EC runoff)-0.88= 1.02(nutes absorbed by plant)

Might be easier to determine by changing ec to tds measurement. Then if you have a tds of feed tds of 1000 and a runoff tds of 600 then you know she easts about 400ppm between inputs.