When I do this test, the instructions for the Taylor 2006-K test kit say to add the cloudy solution to the tube until the black dot "just disappears" when viewing from the top. Does "just disappears" mean that the dot starts to become difficult to see, or that it stops being visible at all? There seems to be a 20 ppm range between the point at which it begins to become difficult to see the the dot, and the point at which you can't see the dot at all.