I don't know C++ or Java, but I was able to find the formula Dave Durant used in his excellent Profilinator code. I've used his formula to build a calculator on my website. I put his formula into a spreadsheet, which is included below if you want to see what goes into it. That's the exact formula I'm using on my site. I've updated the program since taking that screenshot, so it will look a little bit different (read: Better!)