You might want to try this one:

http://www.csgnetwork.com/e6bcalc.html

I adapted it for my Sony-Ericsson k610i screen and it works like a
charm. I put links to the various sections at the top so I don't have to
scroll so much, and got rid of a lot of the table formatting. All the
Javascript is self contained in the page itself, so you can save it
locally on the phone.