To measure NO3 from KNO3:
Lamotte is about the best trade off for decent results, ease of use, runs 50$.
Refills fairly reasonable however.
Still, with all NO3 and PO4 test kits, you MUST calibrate them to see if the ppm's are accurate. This is the same reason why they calibrate pH monitors/meters routine.
You have no way of knowing(in otherwords, you are guessing) whether the reading is correct unless you compare it to a standard known reference.
Here's a semi easy way to do it:
You can also use a scale to 2-3 decimal places and do this also.
VERY few aquarist bother to do this, but plenty of hobbyists tell others to test these parameters, generally forgetting, or simply not knowing how to calibrate or even that they need to, they just assume the readings are correct.
Getting aquarist to do this step is like pulling teeth as you can image, particularly with folks not familiar with KNO3 etc and all the other stuff.
Water changes are much easier to explain and get folks to do, so that and experience knowing when to do a water change is a much easier sell to planted aquarists. Then you dose known amounts(nothing runs out), and change the water(nothing builds up).
Much easier for 99% of folks.