Given a screen of dimensions AxB, and a button or other UI element of size CxD, what are the best methods of calculating a rough gauge of how easy it is to click/reach/target that interface element?
As Roger pointed out in a comment, it looks like it is Fitt's Law your going for. It is possible to try it in this web based test. At the end you get the result and the way it's been calculated. Try it out:
You make 50 hits of different distance and width looking like this when in test: