1. rule of thumb - Noun
2. rule of thumb - Phrase
a rule or principle that provides guidance to appropriate behavior
Source: WordNetrule-of-thumb
And of course we are familiar with the English common law rule of thumb that said a man could in fact use a stick no bigger than his thumb to discipline his wife and family. Patricia Ireland
A rule of thumb with humor; if you worry that you might be going too far, you have already not gone far enough. If everybody laughs, you have failed. Christopher Hitchens
Jesus offers his disciples a simple rule of thumb which will enable even the least sophisticated of them to tell whether his intercourse with others is on the right lines or not. All he need do is to say "I” instead of "Thou,” and put himself in the other man's place. Dietrich Bonhoeffer
A good rule of thumb is every couple of hours take fifteen minutes off. Naps are also very helpful, short naps. Even a ten or fifteen minute nap in the middle of the day can be the equivalent of an hour and a half of extra sleep the night before, and it can raise your effective IQ by ten points. Daniel Levitin
[...]I should say that it is a good rule of thumb never to mention religion if you can possibly avoid it. George Orwell
A good rule of thumb. Traditional Proverb