You'll need a couple of things.
Weights: These should be a set of 3 a LOW, a MEDIUM & a HIGH.
What's low medium & high? it depends on the wrench's range, but should be one almost at the bottom, one as close as possible to the middle & one high enough to be a similar amount away from MIDDLE as LOW is, but in the opposite direction.
Now you need a suitable way of hanging them, wire is pretty good because it's both flexible & strong & can be made into loops easily.
Next you need a strong bar over 1 Ft long, it needs to be strong enough to hold whatever you chose as HIGH horizontally with safety.
What you do is weld a socket to the bar, measure
exactly one foot, & make a detent. Now you set the wrench, clamp it (or hold it depending on the weights) attach the 1 foot bar
HORIOZONTALLY (this is important) & carefully hook on the weight using the cable or whatever as an attachment method. Each of the weights should be done 3 times. 1 is set slightly HIGHER on the wrench, it should indicate so however it indicates. then set dead right & finally set slightly high.
Repeat for the other 2 weights.
This should indicate both the LOW MIDDLE & HIGH offsets, if any, & allow you to graph a slope to show any corrections across the whole range.
The wrench is now set up for Lbs weight at Ft distance
ie foot/pounds. You can use any converter to translate to newtons or whatever you need.