Some skill is required. Don't be discouraged that the center finder didn't instantly tell you where the edges are. Go play with it some more. Good skills do not instantly happen. It might take 10 to 15 minutes to locate the edges until you get the hang of it. Here is a link from the Tormach site that might help; <http://www.youtube.com/watch?v=f0od-cp_9dg&feature=youtube_gdata>
You can always throw money at this problem and buy one of these: <probe cnc,cmm | eBay.