Rule of thumb - Wikipedia, the free encyclopedia
"A rule of thumb is a principle with broad application that is not intended to be strictly accurate or reliable for every situation. It is an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination."
It's basically a "rough rule"; an approximate rule that gives you a basis to work from. As my mother used to say, it will give you an answer "good enough for gravy" and if anyone knows the origin of that phrase, I'd love to know!
Student or Learner