My morning chlorine level was at 7.5ppm, so that seems like a reasonable amount of chlorine loss, especially if there might have been anything trying to bloom when the chlorine dropped low. We're expected to hit 90 degrees for the first time this year, so I'll be watching it like a hawk and probably doing a second test this evening to see how it's changed over the day.

I think I'll be going back to the pool store for 12.5% after I use up the remaining 3 gallon jugs I bought at Menards. I'm not even sure that the jugs I used previously were 10%, considering how ineffective at least one of them was. Roughly 2.5 gallons out of Midwest's 5 gallon jugs brought me from .5ppm to 5ppm 2 days later (and both measurements were done at Midwest, so who knows how accurate those were.)

This brings up a question I've been wondering about the "Best Guess" method: assuming a pool with 40 CYA, is <4 ppm FC pretty much equal to a pool with 0ppm CYA and 0 FC? If this isn't clear, does the CYA effectively bind up a lot of the chlorine such that it's there, but not always available until the CYA-chlorine bond separates? I guess I'm just curious why there is a minimum of 3ppm when CYA is between 30-50ppm?