After getting tired of manually doing the maths for every box I was creating when converting a PSD to a web page I decided to make an extension to do this for me.
The script is triggered using `rc` and requires two parameters. The first should be the with of the item you want to calculate the percentage width for and the other should be the width of the container. The default container width is 980px.
For example, for a 300px wide div in a 900px wide container you would type `rc 300 900` at which point the script would return `33.33%`.
Download from my site: http://danielgroves.net/notebook/2013/04/alfred-extensions/