The site Rules Of Thumb defines a rule of thumb as a homemade recipe for making a guess -- an easy-to-remember guide that falls somewhere between a mathematical formula and a shot in the dark. They help you appraise a problem or situation. They make it easier to consider the subtleties of the topic at hand; they give you a feel for a subject. And they make excellent cocktail party fodder.
Check out: www.RulesOfThumb.org