the rule of thumb
先程と同じニュース中から表現を見てみます。
a [the] rule of thumbに着目しました。ここでは「経験に基づく方法」(『ジーニアス英和辞典 第3版』大修館)という定義になり、Cambridge Dictionaries Onlineには’ a practical and approximate way of doing or measuring something’と記載されていました。(Shou-VR*)
"Get out of the city and the lights to give yourself a chance to see them," Bakich says. The rule of thumb is that you should be able to see all the stars of the Big Dipper — seven stars if you are counting — to give yourself enough darkness to catch the shooting stars. And give your eyes an hour to adjust.