Rule of thumb says
The phrase Rule of thumb says has been in circulation since the s. Where does "rule of thumb" come from? A modern folk etymology   relates the phrase to domestic violence via an alleged rule under English law that allowed for wife beating provided the implement used was a rod or stick no thicker than a man's thumb. Rule of thumb says rough and useful principle or method, based on experience rather than precisely accurate measures. If a rule of thumb is utilized to develop a range of value for a small closely held business, one should be careful to understand what the rule of thumb proports to value.