I've been having fits testing CYA ever since I've been doing BBB.
I never felt good about the readings and thought the test was too subjective.
I finally broke down and bought the standardized 50ppm sample.
I've been playing with it all afternoon and found that if I hold the test beaker at chest level, rather than waist level, I can nail it every time.
When I did it this way with my pool water I found I have about 25-30ppm rather than the 50 I thought I had.
I've been using more chlorine than seems right. I'm going to add about 2 lbs of stabilizer this afternoon.
You might find you need to test at waist level, under your chin or down around your knees. It depends on your particular vision.
I highly recommend getting the standard solution. It will show you the right way to test...IMO