I respectfully disagree, K does not block nutrient uptake in small amounts like 36 ppm (or even way higher concentrations). The recommended levels on the fertilator are mere guesses at what the K level should be, and do not indicate harmful levels at all. This is certainly not the case with other nutrients like Ca, Mg, N, etc... these nutrients will cause nutrient uptake problems if dosed in too high a concentration.
I started getting K blocking problems at about 500 ppm concentrations.
Also, K is not an easy element to test for, and is notoriously hard to measure accurately, so any test kits that are cheaply available for K (if there are any) are going to be very inaccurate. I took my water samples to a lab and had them vaporized and tested the spectral emissions.
There are only a few plants that are sensitive to higher K levels (1 or 2, by my count) and these are not very commonly kept.
It is always best to use test kits to determine the nutrient levels in our aquariums and dose accordingly like you suggested, but the test kits available for potassium won't be able to do this. I believe the best course of action is to read the plant signs and dose a relatively modest amount of K to ensure they get enough.