From James J. Hill Reference Library:
Per the website, "A rule of thumb is a homemade recipe for making a guess. It is an easy-to-remember guide that falls somewhere between a mathematical formula and a shot in the dark. Rules of thumb are a kind of tool. They help you appraise a problem or situation."
Rulesofthumb.org aims to be "the Google of guesswork" - a place to help people make an educated guess on a particular problem at hand, whatever it may be.