a rule of thumb

a rule of thumb

a way of calculating something which is not exact but which will help you to be correct enough
Usage notes: A rule of thumb was originally a way of measuring using the width or length of your thumb.
A good rule of thumb is to cook two handfuls of rice per person.
See also: of, rule, thumb
References in periodicals archive ?
If a rule of thumb is utilized to develop a range of value for a small closely held business, one should be careful to understand what the rule of thumb proports to value.
Everyone's financial situation is different, requiring a more thorough assessment than a rule of thumb can deliver in order to ensure financial protection for loved ones.