Knowing by experience. The toughest thing to explain to someone.
That reminds me of an old joke.... real old.
@WildBill might appreciate this one.
Back in the days of the house phone and having to pay for long distance. So imagine if you will......
Wealthy guy has a Ferrari. He needs an engine service and has a great mechanic. They are finishing up and as everything is going back together they realize he hasn't got the torque specs for the head bolts. What to do?
They manage to make a long distance call to Italy. It takes several tries and staying up late into the night to try and get Maranello on the line. Eventually with broken Italian and English they get the head mechanic on the phone. They ask him what the torque spec is on these highly important bolts. After several hundred dollars of phone time the head mechanic says:
"The torque settings, lives in the arm of a good mechanic." And hangs up.
HA!