But how do I calibrate my luggage scale?
use the torque wrench 
I might try this with my torque wrenches. Don’t know what I’m likely to achieve but hey ho they’ve never been calibrated since leaving the factory and for one of them that’s coming up for half a century.
Luggage scale (analogue), vice, socket, tape measure, masking tape, pencil - check
a round tuit - …
Of course with more than one torque wrench you could always check one wrench against the other by using high low settings and then visa versa before checking them against the luggage scale. What that will prove is unclear but seems a logical step and there may be some emerging pattern.
Now where did I leave that round tuit?