What is a Rule of Thumb?

The exact origin of the phrase "rule of thumb" is uncertain.   There's the mostly fictional medieval legend that a man was allowed to beat his wife so long as he used a stick no thicker than his thumb.  Another suggestion is that, in many languages, the words used for "thumb" and "inch" are either the same word or two very similar words.  This interpretation would assume that everyone's thumb is an inch long or very close to it. 

The one that got my attention is that in history, wood workers -many of them very skilled, mind you- used their thumbs as a measuring device "cementing its modern use as an imprecise yet reliable and convenient standard."  They did this due to the lack of the more modern and very precise tools that wood workers used today.  So, my question to you is this: Do you want to build your financial dreams around something that is "imprecise yet reliable and convenient"?  What if there are new tools available to allow you to be precise?  Wouldn't you rather rely on those tools instead of  "everyone's thumb is an inch long or thereabouts"?