Live Demo at http://wvrzel.github.io/GetPercByCircles/
GetPercByCircles is a small javascript-css libary which offers a friendly circle / pie-chart shaped userinterface for setting percentage values.
There are three supported colors: blue, green and red.
If you put serveral of these circles on one page, they will prevent the user from choosing values, so that the total sum would be > 100 %.
Have fun.
PS: Thanks to lugolabs for inspiration ( https://github.com/lugolabs/circles ), Itay Gal for code ( http://stackoverflow.com/a/21198231 ) and jQuery for everything else.