How to calculate how clickable a button or object is on screen

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?

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Quick comment: - screen resolution is not a good measure these days - use physical dimensions, and consider the type of device and the way it's held. – Roger Attrill May 9 '12 at 13:20
I'm guessing Fitts's Law and the Shannon formulation is appropriate here? – Roger Attrill May 9 '12 at 13:25
Sounds like you're basically asking for Fitts' Law, or are you asking about something more specific? What's your definition of "clickable"? Error rate, time to click? – Ben Brocka May 9 '12 at 13:39
Well I had the idea of colour coding elements on a screen based on how easy they were to hit as a vague rough guide, but for that I would need a rough metric, so something like Fitts law could work yes – Tom J Nowell May 9 '12 at 13:55
There are a few cases where Fitts' Law doesn't quite fit (sorry I couldn't help myself). A very small target (e.x. 10px by 10px) might be hard to easily hit anywhere in the middle of a screen. If you take that same target and place it at the edge or corner of a window, the target immediately becomes infinitely wide/tall as the cursor is unable to overshoot the target due to the boundaries stopping it. – zzzzBov May 9 '12 at 19:55

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:

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