The version of "Rule of thumb" that I heard was that a man was allowed to beat his wife with a rod no thicker than his thumb.
Actually that rings a bell somewhere. I stand corrected if I'm wrong. My idea is for the romantics amongst us ... Yours is for the realists :-)
A shortened form of ruler is 'rule' "Use the rule". A male thumb is an inch wide (try it). Measuring a short distance thumb to thumb is the 'rule of thumb'. It means it's rough but better than nothing.