I have really mixed feelings about test methods. One thing that I find really overlooked is getting your methods and eyes set to get a consistent reading. I finally set up containers of various nitrate and phosphate of known levels. Running tests with the API liquid, I got totally worthless nitrate tests as they all looked alike to my eyes. With known samples, I now use the cheap Jungle brand 5-in-1 strips I get at Wal-mart for most testing but for phosphate, I use the API.
I find so many ways to make the liquid tests miss that I am reluctant to tell new testers to use them. Not shaking, not holding the bottle correctly, not timing it , lighting changes, can all throw the readings off so I often feel the strips are more likely to get a ball park answer better than the liquids.
But either method has to be done in a consistent way to get it close and that often means you need some samples of known values to find what the color will look like to your eyes under your lights. It does take some real practice, though. Your eyes and my eyes will likely read the colors differently.