Umm, since he's worried about biofilms, I wouldn't recommend a low chlorine routine at all.
Go high, and just dump and refill more often to cover the CYA problem.
btw, I notice a slight cough problem from my tub too. But it hasn't returned since I quit worrying about nailing the numbers exactly. These days, I just throw in a tablespoon of dichlor a day, period, and shock once a week with MPS.
Sure it's more than needed, but chlorine and water are cheap.