A practical principle that comes from the wisdom of experience and is usually but not always valid: “When playing baseball, a good rule of thumb is to put your best hitter fourth in the batting order.”
A rough and useful principle or method, based on experience rather than precisely accurate measures. For example, His work with the youth group is largely by rule of thumb. This expression alludes to making rough estimates of measurements by using one's thumb.
[ Second half of 1600s