Rules of thumb are common guides to navigating everyday life without having to refer to some authority. Wickipedia says the term is thought to originate with carpenters who used the length of the tip of their thumb (approximately 1 inch) as a handy – get it, handy – unit of measure rather than resorting to a ruler. In general, rules of thumb are principles with broad application not intended to be strictly accurate or reliable for every situation. They are easily learned and easily applied for approximately calculating or recalling some value, or for making some determination.
Farmers have many rules of thumb. One that I remember and invoked today was “Plant you potatoes when dandelions bloom in an open field”. The logic is that the weather (and soil) is warm enough for the seed potatoes to flourish. Why in an “open field”? Because the soil near a foundation is warmer than the soil in a field and dandelions will bloom near a foundation days or weeks earlier than those in a open field where the soil will still be too cool.