I have learned the hard way. Use a torque wrench, especially when inserting a steel fastener into aluminum, for example, spark plugs. Cheap insurance.
On another forum, a professional aircraft mechanic did two studies. One was to test the "elbow calibration" of experienced mechanics against a torque meter. All failed, by margins up to 25%. In another study, he tested Harbor Freight 1/2", 3/8" and 1/4" click type torque wrenches against the same meter. They were within 5%, pretty good for $11.99 wrenches. I have tested them myself, and they are more accurate than my Craftsman torque wrenches. I have one of each, plus, if they ever tear up or malfunction, take it back and get another one.
John, Medina, TN - 2003 Honda ST1300 ABS, 2011 Kawasaki KLR 650, 2012 Ducati Monster 1100 EVO
2017 HD Road Glide Special