The idea is just to start out with what is the most likely value -- that claimed by the manufacturer. As I wrote, it's not that important, but if you're going to have an automatic calculation you might as well start with the correct intended number. Errors accumulate so there's no point in starting with something you know is likely to be wrong.

6% Bleach doesn't degrade very fast because the rate of degradation goes roughly as the square of the concentration. 12.5% Chlorinating liquid degrades more quickly. See the chart at the bottom of this page. As for impurities, you'll note that this manufacturer does remove catalyst metals from their source water and I checked with Hasa on the West Coast who is the source of the chlorinating liquid I use as well and I also checked with Clorox about their bleach. Most of the big players take care to produce a quality product. I can't speak for the off-brand Ultra bleaches.