Usually the trap ring (or in this case... ball) would be touching or nearly touching the rail to get the crew to trapeze as low as possible. From there it is up to how comfortable you are on the wire. Higher can make it easier to come in off the wire, but higher can also be harder to hook into. You might have to lift your butt to get the ball into the spreader.

These balls and rings are normally hung from a wire that ends about 32" above the deck with a "T" handle at that point so you can lift your weight on and off the wire. From that point there should be a line system that is also attached to a bunjee to pull the ball up when not used. There is normally a "rope lock" on the line to adjust the height of the ball / ring. The bunjee runs under the tramp to the opposite trap system.