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Rule of thumb
A rule of thumb is a principle with broad application that is not intended to be strictly accurate or reliable for every situation. It is an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination. Compare this to heuristic, a similar concept used in mathematical discourse, or in computer science, particularly in algorithm design. See also mnemonic.
Does rule of thumb mean approximately? I thought it meant "in most cases" or "as a rule."
To keep it simple, it means approximately.
Craftsmen use their bodies for measurements, so a rule of thumb [measurement using a thumb] is an approximate measurement compared to the exact measurements imposed by the metric or imperial systems.
rule of thumb 
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