Did you have any success with measuring lower CYA levels in the meantime?
Being home a lot due to Covid, I ran a few tests. I ordered a glass cylinder that's a bit longer than the test tube than came with the test kit. I copied the 30 to 100 scale from my test tube to the cylinder, and painted a black dot at the bottom.
Then I created a 100ppm reference sample using a precision scale. That sample I then diluted down to create references at 50 and 30, and used those three samples to verify the scale on my cylinder. Was pretty accurate. You could also order the 50ppm reference from tftestkits, but I liked to have the 100ppm reference as well.
Then I diluted my 100ppm reference further down to 20, and used that to extend the scale on my cylinder to 20, about 2.5cm above the 30ppm line.
For my purposes that's all I need. I just want to be able to maintain CYA around 30 over winter. At low temperatures, CYA binds more chlorine, so I prefer to have lower CYA while the water is cold. But I also don't want it to get too low, at some point the HOCl level would explode. I keep my FC around 9-10 over winter. I don't want to risk bleaching my equipment should CYA drop towards 0 at that FC level. And I also don't want it to take too long to get back to summer levels when temperatures start to rise again.
With my tube I get a good guess if I'm closer to 20 or 30, and find the right CYA dose to bring it back to 30 without overshooting.
The reading gets a lot more inaccurate beyond 30. With that much liquid in the cylinder, the dot is starting to look very small. I guess there is a reason why the standard test only goes to 30.
It helps me to have a black dot that's not round. When turning the tube, an irregular shaped dot is easier to spot. The test is quite tricky for me to start with. I am wearing multifocal glasses that I have to take off for the test, and then move the test tube up and down in front of my eyes to make sure the dot moves through that sweet spot where my eyes can actually focus. Having an irregular dot makes it a lot easier for me to be sure that I can't see it.
If you need it more accurate, then you'd probably have to look into getting a turbidity meter. From what I've read in other posts, they seem to start at about 500USD. Too much for my purposes.