Ya switch your PO4 and NO3 to salifert, I think much more accurate.
This was my tank's NO3 reading back in Jan. This showing in high resolution mode so all reading have to divide by 10. You see the increment is from 0 to 0.1 then 0.2, 0.5, 1ppm (under high res mode)
So If you really get a clear color reading then you're sure it is actually 0ppm. As opposed to having the scales at 0 then the next one 12.5 so if you get a clear color it can fall somewhere between 0 and 12.5ppm.