Determining visible area/invisible area of a masked object

I have a movie c开发者_开发百科lip named circle_mc(see the image on the link below) and a mask object(movie clip) which determines visible area of the circle. User can change the portion which is visible (extend the red area and make it red+blue, gray area is invisible). I need to find proportion of visible area to invisible area (red+blue/gray) so that I can calculate angle.

Here is a working example of a formula.