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)

or

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.

Ran a runoff from FFHF test this evening this is what I got. From my understanding these are good numbers Am I correct?

PH: 5.6

EC: 1.7

PPM 500: 880

PPM 700: 1230

Fox farms happy frog has had some issues with PH being very low out of the bag… been giving new growers trouble.

pH is off. The pH range for soil is 6.3 to 6.8.

Okay I hope I transposed the PH numbers, I’ll water to runoff again in four +/- days and double check it. Thank you all for the reply’s!