But which do you think is more accurate: the expensive test kit, or the calculated amount of pure potassium compound that you put in the tank? I would trust my calculation any day versus what a test kit told me.
That's whole jist of Plantbrain's idea. Don't trust the test kit! Add the nutrients as if the test kit was not telling the whole truth, and then reset the ingredients weekly with a water change.
My Hach TPTZ Fe test kit is supposed to be very good, but it never agrees with what I put in the tank. I think it is better to go with the "empirical" method. Calculate the dose accurately, measure the dose accurately, and just put it in the tank.
I would use the Fe test kit to check the difference in doses, say, if I went from .2 ppm Fe to 2 ppm Fe, as an experiment, but I would still trust my calculation more.
Just my two cents. Actually I am envious of you having the kit.
I feel your overall experience (and your results) in this hobby far exceed mine. But I would be using the kits for checking orders of magnitude, not a 5.5 ppm vs. 10 ppm difference, which I think is not too meaningful. Unless you doubt the math you used to calculate the dosage. In that case I can calculate it for you.