Here is a calculator that we like better:

http://www.poolforum.com/zxq/BleachCalc262.exe

It is more than just a bleach calculator. If you click on the drop down menu in the upper left-hand corner, there are other calcs.

However, Jason is right that you add a smaller amount than you think you need, test, add more if needed. The exception to this is bleach. When adding bleach, you need to add enough to get to the chlorine level that will be high enough to sanitize your pool.