the.com/rule of thumb

means A practical, approximate method or guideline based on experience rather than precise measurement or theory.

from The real origin is murky: it likely comes from old craft practices where workers used the width or length of a thumb as a rough, handy unit of measurement — close enough for carpentry or brewing, no ruler required. A persistent folk tale claims it referred to a legal limit on the width of a stick a man could use to beat his wife, but this has no basis in actual law or recorded origin — historians have never found such a statute, and the phrase shows no early connection to that story. The measuring-thumb explanation, while not iron-clad, fits the phrase's long use as a synonym for 'approximate but workable.'