I think Butterfly is alluding to the possibility that in a week, your CYA reading could be much higher. Going from 40 to 0, then back up to 60 after adding CYA a day earlier is slightly suspect. It also doesn't dissolve well when broadcasted, the sock method works very well however (old sock, rubber band, hang from ladder, let soak, squish manually).
I still maintain that I was able to add 20ppm CYA, squish it all out over the course of 2-3 hours and I got an accurate reading the next day. I haven't added any CYA since, and all readings after that are consistent (it's been weeks).
So, yeah, you could have an accurate reading but the general rule for CYA is that it takes up to a week to register properly.
Exactly how much CYA did you add anyway? Did you add 60ppm worth or less? Because if you were at 0, you'd have had to added 60 whole ppm to get to 60. If you added less, your pool wasn't really at 0. And no, the general rule of thumb is that CYA does not go away unless you drain and refill. Rain will dilute it, but gradually. It'd have to rain your entire pool out and displace every bit of the current water to dilute it all the way to 0 (never happen, can't displace only CYA water because rain will mix).
