BigJim's method is the sensible way that I also follow. Basically ammonia & nitrites should always be zero, anything else is bad.
It is possible to get accurate results with color change kits if you want to do serial dilution. <This is how "real" labs do those color tests ....> Anyways, you want to know the threshold for color change for a specific test. That usually is the lowest number on the color card. <For my API nitrate, it would be 5.> You take your tank water, and add distilled water until the test no longer read positive, and calculate the result from the dilution. <Eg. if 9:1 dilution is positive & 10:1 dilution is negative in the NO3
test, then your NO3
is 45 (error range +/- 5 ... your threshold).> Only real fanatics (look in some of the plant forums on the net!) do things like this. For the rest of us, ball park figures are perfectly adequate!