CYA level-my test and pool store test

[Poconos]

One thing you need to do with the black dot test is after you mix the reagent with the water sample you have to shake and wait before checking the dot. When I do the CYA test I mix, then do other tests and come back to the CYA test last. I have checked using the recommended method (I think) of observing the dot in sunlight while held at waist level, and also viewing down through the tube looking at fluorescent light reflected off a white paper....no overhead illumination. Results of both viewing methods are close enough for my purposes, better than +- 10 and that's at the 40-60 levels. I'm guessing at the +-10 but certainly no more.
Al