OK, forget the Turbo Shock...

I added 5gal 12.5% and was at 11.4ppm FC after an hour. Added another 3gal, and an hour later the count had shot up to 27.5ppm. Is it reasonable to assume that the first 5gal hadn't fully dissipated by the time of the first test? I reviewed and followed the Taylor video, so I'm pretty sure I'm testing correctly now.

I can see a marked improvement in the color already - it's definitely lightening. I'm letting the levels calm down a bit now (just tested and still have 25.5ppm after another hour). I guess that at these levels I don't want to be adding any CYA yet?