rule of thumb
Pronunciation:
noun
Definitions
A general guideline, rather than a strict rule; an approximate measure or means of reckoning based on experience or common knowledge.
Example: The usual rule of thumb says that to calculate when an investment will double, divide 70 by the interest rate.
(usually hyphenated) Approximated, guesstimated.
Example: I made a quick, rule-of-thumb estimate of the manhours required for the job.