Our experience with hydraulic wrenches, of several makes and models, is that +/- 3% is the industry standard for accuracy related to the output of hydraulic torque wrenches. This means that if a torque wrench is designed to produce 1000 ft lbs at max pressure then the wrench will still be within tolerances if it is actually found to produce anywhere from 970 – 1030 ft lbs during certification. However, whenever a certification is being performed, the design tolerances of the tool should be confirmed if there is any question.