The calculator in the current firmware 1.1.4 is not good. We will have a better one in the upcoming iPhone 2.0. While we are waiting for the new firmware, Installer has a program called “GoodCalculator” in the “Productivity” category. It looks similar to the default calculator, but it comes with more features. You can calculate +/- and % value easily. If you typed any incorrect numbers, click the “<-” icon. It is slightly better than the default, right?