Take a look at the Best Guess chart in my signature below. With a CYA of 40, you'll want to maintain your chlorine levels between 3-6 ALL the time. If you drop below 3ppm, you risk an algae bloom. So, each evening, add enough bleach to get back to around 6. If you find at the next evening's testing that you have dropped below 3, maybe go just a little higher for the evening dose. (Maybe try 7.)