The phrase "a rule of thumb" is often used to describe a general principle or rule that is not necessarily based on scientific evidence or precise measurements. It is a way of saying that something is generally true or accepted, but not always exact. For example, you might say that "as a rule of thumb, you should drink eight glasses of water a day" or that "a rule of thumb for tipping is to give 15-20% of the bill." In these cases, the phrases are used to indicate that these are general guidelines, not hard and fast rules.