The calc tells you how much to add to get to a certain ppm goal.

Say you input your pool gallons (assuming you set the default so it's not metric, right? ) You test your CL, and you need to add 2 ppm of chlorine. Using the calc, you put in 2 ppm and the strength/% of the bleach you plan to use (like ultra clorox is 6%) and the calc will tell you how much you need to add to get 2 ppm in your pool.