I have a new Taylor K-2006 kit, and tried doing my own CYA tests this weekend. I do find it hard to know how much of the black dot must be obscured to say that it has "disappeared". For instance, it pretty much disappeared at about the same level reported by one of the pool companies I've taken water samples to, but I *could* still manage to see a very faint outline of a black dot (because I knew a black dot was at the bottom). Even after filling up the tube with the remaining CYA fluid I could still kinda-sorta see a black dot at the bottom of the tube, although it was quite hard to do so. So, unlike the FAS-DPD chlorine test where "colorless" means absolutely without any color at all, I'm interpreting the black dot test to be mean that you stop when you can't easily see a black dot anymore. At the least, I'm able to replicate the pool store's numbers with a home-test kit.