Just so we have a real-world data point, I measured the chlorine rise in my pool after adding what is supposed to be 12.5% chlorinating liquid and the FAS-DPD measured chlorine level I got was a little less than expected (4 cups to 16,000 gallons gave an increase of 1.8 ppm instead of the expected 2.2 ppm), about equivalent to what 10.5% chlorinating liquid would do. Given the 0.2 ppm FAS-DPD error, this isn't bad and it could be higher than 10.5%. Also, the pool store where I get the chlorine isn't great about keeping the chlorine out of the sun (it's mostly out of the sun, but is in sun during part of the day) and I don't know their turnover rate.

Richard