HTML5: Get click position from canvas arc

Question

See this jsFiddle post for a working arc drawing; thanks to Simon Sarris for the fix in my previous questions.

I'm using the KineticJS plugin to create shapes and make use of event handlers. Assuming you clicked somewhere on the arc and the arc knew where you clicked (x, y), how could those 2 coordinates be used to determine a percentage?

When you click anywhere, the total percentage is always 100%.

Addin

To make this simpler, what could I do to (x, y) to virtually bend the object so that x goes from 0 to maximum x?