Clark, I was taught as your father, as the load on the wrench is a moment arm. If you have a rod, with a socket attached, so as to mount it to the wrench with, you have to weigh it, and measure the length from the center of gravity, to the center of the socket, in inches, to get the moment calculated for the arm, so you know how big to make the weight, or how much to add to each setting. At that, the arm has to be sticking out from the wrench, parallel to the floor, or level, with the weight acting at 90 degrees to it, and be adjustable along the rod by an inch or metric scale, unless it is a fixed value.
An example, would be, that the moment arm was a metal rod, 12" long, from the center of the socket, to it's end, and weighed 2 pounds. Then, when checked, the rod balances, (its center of gravity or C.G.), at 6" out. You simply multiply the two, to get 12 inch pounds, (6" x 2 Lb.), which will have to be added to the torque from the weight you will add on the rod, to slide along it. If the sliding weight weighed 5 pounds, and its C.G. was at the end of the rod, that would be 5 lb. x 12", or 60 inch pounds, but you still have to add the 12 inch pounds from the rod, so it will really be 72 inch pounds. It's the same as mounting two moment arms on the same axis, and adding the two together to get the total.
The above would be for a pendulum type calibrator.
The other calibrators, that I've seen, have a strain gauge in them, with a digital readout, that you bolt to a workbench. You just watch for the wrenches torque setting, in In-Lb or Ft-Lb, to come up on the LED readout, as you turn the wrench, and see if the wrench will click when it gets to the proper torque. The thing is, the calibrator has to be calibrated and re-calibrated, too, if you want it really accurate. To me, the simple pendulum type would actually be more accurate, for a longer time, once every moment is calculated, and added up. Especially a fixed value pendulum, except you can't adjust these until the wrench clicks, and read the measurement off a scale.
That wrench wasn't that accurate, then, as I figured it would be, if new. However, you do have the correct idea about checking one, as long as you know the measurement, from the center of the socket, to the C.G. of the weight. Make sure the wrench is level, and the weight known. Also, to do it your way, you need to weigh the wrench, and find its C.G., to know it's own moment. You'll have to add the wrenches moment to the weights moment, as that could be throwing your calculation and test way off on the low end.