I hate to sound like I'm contradicting PoolDoc, but, I do see a value (beyond my itchy brain-scalp-very itchy) to figuring how much chlorine will be needed and that's how much to buy. The logistics of buying, moving, and storing enough chlorine for an algae / ammonia clan-up may be well worth considering; especially for us weekend warriors who can only collect the materials on Saturdays.
My two cents