If you take a look at the Best Guess Chlorine Chart below, you can see that for a pool with a CYA of 40 needs to have chlorine levels between 3-6 all the time and if you are fighting algae or cloudy water, then your shock level would be 15.

In a 13,000 gallon pool, each quart of 6% bleach will add just a little over 1ppm of chlorine. So, each evening, test your chlorine level and add enough bleach to raise it back up to 6ppm. The next evening, if you find that you have dropped below 3pm, then you'll know you need to maybe take the chlorine to 7. You want to be able to make it through an entire day without it dropping below 3ppm.

Also, check your pH daily and keep it between 7.2-7.8.

You might find the bleach calculator helpful. Look in the Forum Q & A section near the top of the forum homepage. One of the first few threads is called "bleach calculator" which has the link you can download to your computer. It actually has more than just the bleach calculator. If you click in the upper left hand corner, it will drop down a list of other available calculators you can use.