Go to http://www.musiccalculator.com/
Download the offline version, uncompress it and place it somewhere where you can find it easily (eg. your Documents folder).
Open the MusicCalculator folder and launch index.html in Safari. Go to File > Open in Dashboard..., then highlight the calculator. Make your selection a little longer downwards because some of the functions inside Note can get a bit long. Click Add.
The Music Calculator should now show up in your Dashboard, which is one keystroke away any time you need it. I found it very useful working on an Ableton and turntable mix.
![Image](http://www.unit27music.com/wp-content/gallery/tutorials/musiccalculator.png)