You don't calculate what your existing PPM is, you measure it with a test kit. Measure the FC with the DPD-FAS test. If you want to get to 20 ppm Cl, and your test results are 3.5, then 20 - 3.5 = 16.5. Put that number in the ppm increase field and hit calculate. You'll be shown how much bleach to add.

Michael