Can you help me with some portable physics?

I’m working on a prototype deer stand safety harness that would use a rope and some counterweights to lower the person in question from his tree stand to the ground. I need a formula to calculate how much weight I’m going to need to offset a falling hunter while avoiding any sudden stops.

The plan is to secure the harness to a climbing rope (1200lbs test/120lbs working load, which seems like it ought to be enough), pass the rope through a carabiner strapped to a tree trunk, to a set of weights probably kettle bells in the prototype on the ground.

Don’t look at me like that. I took chem and bio in high school, not physics.