Shock level for a pool with a CYA of 70 is 20ppm. In a 21K gallon pool, each gallon (4 quarts) of 6% bleach will add about 3ppm of chlorine. Each gallon of 8.25% bleach will add 4ppm. Use that info to help you figure out how much chlorine to add each time you test to get the chlorine level back up to 20.