so I found a video http://www.youtube.com/watch?v=XLluimXmhS4
you have to watch both part 1 & 2
I like this video because he makes a mistake and then trouble shoots it. It's a very simple calculator but it gives you the basics to go from there.
I haven't gotten all the kinks out, but thought I would post because I'm sure others are anixious to find answers.