I like this idea better than another one I saw where you make holes on the saw. Just a few changes thought. Maybe make a slot on the board, or multiple holes for different size circles. Also I like one commenter that mentioned just slowly raising the blade up so you don’t have to constantly be cutting off bits. Oh and some hold downs too would be nice. Oh and maybe use a bolt instead of a woodscrew to hold the board on.
http://www.instructables.com/id/How-to-Cut-Perfect-Circles-with-A-Table-Saw/