But now we need to calibrate the weight, how do we know it is 1 1/4kg? Is the tape measure accurate? Is the square square? Are we pulling on the horizontal? Does the webbing strap have any stretch in it? The variables are endless.
I am joking, obviously
Compare it to another wrench? Or look up how to test a torque wrench, using a fixed weight.
Not sure how to do 5nm without looking it up in a reference book.