Or this?

The real explanation of 'rule-of-thumb' is that is derives from wood
workers (or other constructors) who knew their trade so well they rarely
or never fell back on the use of such things as rulers. Instead they'd
measure things by, for example, the length of their thumb; they measured,
not by a rule(r) of wood, but by rule of thumb. The term was already in
metaphorical use by the late 17th century.