Jeff Atwood of Coding Horror has a great writeup today on Fitts’s Law, which can help you conceptualize the amount of time needed to interact with a user interface (which doesn’t need to be the user interface of a computer; it applies just about everywhere). Jeff also does a great job of explaining some of the issues that occur when Fitts’s Law is naively applied in every situation.

The most important takeaway you can have from the full discussion is to maximize the utility of the natural borders of your screen. There are a ton of resources out there on Fitts’s Law, and it’s exceedingly useful to understand it well if you are doing any UI development.