Rule Of Thumb Meaning

(attributive, usually hyphenated) Approximated, guesstimated.

Example: A general guideline, rather than a strict rule; an approximate measure or means of reckoning based on experience or common knowledge.
  The usual rule of thumb says that to calculate when an investment will double, divide 70 by the interest rate.
c. 1935, Ogden Nash, "Reflection on Ingenuity" in Verses from 1929 On (1959):
  Here's a good rule of thumb:
  Too clever is dumb.
  I made a quick, rule-of-thumb estimate of the manhours required for the job.