You can check/calibrate the wrench by clamping the drive in a vice and a known weight hanging off the handle at exactly 12 " from drive C/L ..[ if it is a ft/lbs wrench]...
Wrench must be parallel to floor and the weight Total must include the hanger.
Best accuracy is attained when wieght is approx. 1/2 way point of wrench range specs.

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A Dalton