If your system is producing a score for each of the sides, you can produce a graph mapping how often teams win given your predicted margin between the two. Then use Excel's Solver function to discover the curve that best describes the relationship. Now, for future games, when you determine the margin is 10 points, you can use the formula of the curve to say that your predicted winning team has a 60% chance of actually winning.

I go into a tiny bit more detail and show a graph in this post: http://www.ozmium.com.au/forums/sho...959&postcount=2