Ok, I looked and didn't find this method. For making something fit a irregular edge you have to have a pattern to cut from. The easiest way I have found is to use a drawing compass. This article explains it better than I can. Lee valley tools sells a megabuck professional layout compass that is fantastic for this sort of thing but a child's circle drawing type works mighty fine too.
http://www.familyhandyman.com/DIY-Proje ... erfect-fit