Just set them by feel. A little drag in the pan and good to go. Get rid of side by side slop if it's bad. It ain't rocket science. I can't imagine having four pounds for fox, some only weigh about 7 lbs, how they gonna set off the trap?
Agreed.
I have never measured pan tension on a trap. For land trapping canines and coon I typically set the pan where it no longer free-falls and needs a bit of pressure to press it down. I can tell by feel where I want it. Nothing scientific and I don't spend a great deal of time worrying about it.
One of the big issues I have with MB550s and the Duke 550s is the pan tension. I personally wish you could adjust the pan tension with a bolt. I am not a fan of bending dogs to try and tweak the tension. Stock they seem fine for coyotes and cats, but IMO have too much tension for fox.