I have been getting inconsistent results like...0, 40, 0, 35, etc... and I now know what the explanation is. When I shake the reagent powder vigorously to be sure of dissolving it all, and then knock out any bubbles, and let it stand for the bubbles to float out before final check I get zero.... so it looks like user error. The powdered reagent needs a bit more effort to fully dissolve than some others like ULR Phosphorus..so something to watch out for.