Yes basically that is what I was using it for, but in different light conditions I see the dot slightly more or less (seems more sunlight higher CYA reading), so I was using it as a reference, when pool sample matched appearance of reference we wold read. This is the hardest test for me because there is some subjectivity to it. Many times I get my wife to read the pool test and we come up with different #, that is when I bring out the reference and it helps. If I stare long enough I seem to many times see the dot so I just try and glance. However in "Extended test directions" it states "Continue pouring until all traces of the black dot at the bottom of the view tube completely disappear, even after you stare at it for several seconds". If I had a reference all the time I would be more accurate. While we are usually within 10 ppm from each other and maybe that is good enough. I now keep FC 3 ppm above target listed in chart when using solar cover, but this will lead to prematurely wearing out solar blanket. Just wondering how long I could save the reference, if at all?