[RonBee]

Originally Posted by banderas

Not really, philadelphia.
A: How much is a portion of rice?
B: A rule of thumb is that it is is two and a half handfuls.


Rule of thumb is "likely that it refers to one of the numerous ways that thumbs have been used to estimate things - judging the alignment or distance of an object by holding the thumb in one's eye-line, the temperature of brews of beer, measurement of an inch from the joint to the nail to the tip, or across the thumb, etc."

Banderas's example is just right. If you use a rule of thumb measurement you are satisfied with an approximate measurement. You don't need exactitude.