Jeremiah
+/-4% (or +/-6% depending on type) is the norm for torque wrench calibration tolerances, there is a British Standard, but the number escapes me. To eliminate variation, the geometry of the test need to be controlled, i.e., the wrench has to be at 90deg to the calibrator, the load needs to be applied slowly over a period of about 5 seconds, no jerking or sudden movements, (this is important), all the equipment need to be very secure. As a point, the digital wrenches are no more accurate than mechanical indicating items. The method in the British Standard says, "exercise" the wrench at 100% of the scale first, 5 times, and then check at 20%, 60% and 100% of the full scale, take the average of 5 readings at each setting as the result. Repeatability of measurements should be in the order of 0.1ft/lb. You may be able to adjust to "best fit" to meet the tolerance, but this equipment does need regularly checking. One of my customers checks on every issue from the stores. Please PM if you need any more information.
Paul