With a cya of 50, you need to keep your chlorine between 3-6 all the time. Test each evening, and each time add enough bleach to get back to 6ppm. In a 30,000 gallon pool, each half gallon (2 qts.) will raise your cl by 1ppm. You can add the bleach slowly to the skimmer while your pump is running.

http://www.poolforum.com/pf2/showthread.php?t=365