I’ve always loved rules of thumb, but if you pressed me to define what they are I’d just flubber out something obtuse.  That’s why I was very pleased to find this on Kevin Kelly’s Cool Tools:

(Tom) Parker has refined his explanation of what rules of thumb are, and why they are cool tools. He writes:
"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. They make it easier to
consider the subtleties of the topic at hand; they give you a feel for
a subject. A rule of thumb is not a joke or a ditty. It is not a
Murphy’s Law. Murphy says that things will take longer than we think; a
rule of thumb says how much longer. While a proverb says that a stitch
in time saves nine, a rule of thumb says to allow one inch of yarn for every stitch on a knitting needle."

Kelly also links to Parker’s new website dedicated to rules of thumb which I think might be one of the most interesting sites I’ve ever come across.  What makes it REALLY cool is that he solicits rules of thumb from readers and then asks other readers to rate the rules so he’s probably going to amass an even greater treasure trove of wisdom in the near future.  Here’s a couple of my favorite rules from just a five minute perusal of the site:

• If you can’t adequately and clearly explain a concept to a neophyte, you don’t understand it clearly enough yourself. —
  Adam, CIO, Perth 
   
• For fatty foods, leave 40 percent of the grill exposed to avoid flareups. —
  Gerri Willis, USA 
   
•   When you’re playing blackjack, assume that any unseen card is an 8.
•   For marketing purposes, elderly consumers think they are 15 years younger than they actually are. —
    Tracy Lux Frances,  Bradenton,  Florida 
   
•   Advertising costs should not drop below 10 percent of sales until a business has been around 20 years. —
    Captain Haggerty,  animal trainer, actor, author, and philosopher,  New York,  New York 
   
•   The year you start growing dark hair on your chest is the year that the loss rate of your head hair exceeds its growth rate. (I must be the exception that proves the rule, because if this was true I’d be bald twice over by now; Jon). —
    Mark Ryan,  Dallas,  Texas 
   
• You are middle aged when your high school and college days are featured
  as nostalgia on TV. You are at old age when your wedding presents are
  sold as antiques. —
    Margaret M. Day,  Locke,  New York 
   
• When forced to estimate an adult woman’s age in her presence, take the
  figure you think she is, divide by two and add 15 (add 20 for a woman
  presumed over 50) —
    Jim Veihdeffer, PR pundit, Phoenix, AZ, US
•   If you can touch the ceiling of your house with the palm of your hand, your ceiling is too low.  — Bob Horton,  consultant and writer,  Largo,  Florida
• If friends ask you to help them move, remember that the work will begin
  an hour after you get there, you’ll finish an hour later than expected,
  the pizza will be colder than the beer, and the beer will be in lesser
  quantities than promised. —Tom Sacco,  West Des Moines,  Iowa
•   It takes as much time to paint the trim in a room as it does to paint the walls and ceiling. — R. A. Heindl,  design engineer,  Euclid,  Ohio