calibrating nitrate test kits

I decided to finally calibrate my test kit due to that I am soon starting up a new tank and figure I'll be doing some testing until I figure out a good dosing regime. But I seem to have created more doubt than I had before. In my current tank I am getting readings around the 10ppm color. Upon searching the first site I found with info was Rex's instruction which is what I did. I setup test kits containing just tap, 5, 10, 15, 20 ppm. After 5 minutes all vials were plain yellow.

So before I go and start re trying, I have been using Spectricide Stump Remover from Lowes for KNO3... it seemed to have been working fine in the tank, and have been getting some readings (unless its just from fish waste). Does anyone know if Spectricide isnt a good source of nitrate? And the instructions say to use 6.5 grams KNO3, is it possible the Spectricide stuff is a different weight per KNO3? It did have a different look/consistency than the KNO3 I got from greg watson (as I recal GW's was more sugar looking, and this stuff was more baby-powderish).

 

Alright, when I get a chance - probably tomorrow - I'll re do the whole thing. For all I know I might have put two bottle #1's in instead of 1 and 2. I know it probably isnt competely junk since I do get somewhat normal readings in the tank... so I am going to figure it was something I did in the process. Just wanted to make sure the KNO3 I was using wasnt the problem. Its the API test kit btw. When I decide to buy a new one though, it might be the Lamotte.