Only test once. Then, add a dose that will take you up to where you need it. (Doses are based on your volume.) Remember that your range for needed chlorine is based on your CYA reading so what StraightLeg needs in his pool may not be what level you need for your pool. (I can't remember what your CYA reading is, Tom.)

For example, in Tom.B's pool, each 121 oz jug of 8.25% bleach will add about 3ppm of chlorine and each quart will add 1ppm.

In StraightLeg's pool, each 121 oz jug will add just slightly under 3ppm. (You both have nearly the same volume.)

Here is a bleach calculator that you may be interested in. http://pool9.net/bleach/. It actually is more than a bleach calculator. There is a drop-down in the upper left corner with other calculators you can use.