I used to calibrate them for the air Force. +-3% was usually the tolerance. Big thing is to back it to the lowest setting when done like TMM said. Also when you set it break it three times to exercise the mechanisms. I found CDI makes them for snap on and order through them half the price. Never really had to re calibrate them usually within limits. Sears sucked and so did proto. We did everything from inch pound pre set screwdrivers to big *** 800 lb ones for aircraft props. Drop any of them on the floor they go out of calibration something to remember. Tell you what a beam style are great unless you bend the pointer. Never had to calibrate any of them at all.
