Get the Salifert nitrate kit. It reads to the 1/10th ppm (or so they claim it to).
I find the kit very reliable. A few months ago I tried dosing trace amounts of nitrate to see if SPS/LPS benefited from it (they did... as did hair algae).
I would add just barely enough calcium nitrate to the system to raise it by 0.3ppm, and sure enough, when I'd test with the Salifert, the reading would jump from 0.0ppm (clear) to about 0.2 (faint pink/clear). I'd say that's plenty close enough for what we do.
Phosphate. A different issue altogether