There are, somewhere on the internet, posts and websites about calibrating your test kit. You create known acids, bases, amounts of nitrates, etc...and see how your test kit reacts to them. Then, even if it's off, you know how much it's off.
There is actually a sticky in the Fertilizers and Water Parameters forum on this site by Hoppy. It shows you how to calibrate Phosphate and Nitrate test kits.