I'd like to get some clarification on how to read the view tube on the cya test. As an example when looking down the view tube I often think the black dot has completely disappeared. But after a few seconds I may start to see a very slight shadow where the dot is. (like I can see it and then I can't and then I see it again etc.) If this was at the 40 ppm line on the view tube by adding more test solution to get to the 30 ppm line usually is enough so I don't see that slight shadow. Would the correct cya reading be 40 ppm or 30 ppm? I think the answer is 30 ppm but since the target FC level is determined by this reading I want to be sure.