tension relative to each other is fine also, as long as you’re in the ballpark at least and not 70 or 150kgf for example.
This is when froggy and I did it on a calibrated tensile tester:
You could get a load cell and make up a frame, or get a digital scale and do the same thing cheaper
Or you could make an even simpler hanging jig and anchor it to a joist and put a known ~100kg on the other end
