The way we set ours up was a steel cable anchored into the concrete, and the other end in a treetop about 500 feet away anchored only about 30 feet lower than the starting spot. The slack in the line is more than enough to drop you a good 40 feet in a hurry, and you want the drop point to be pretty short of the end anchor in case some fuckin retard decides to get scared and not let go.
We learned the hard way. Had the cable barely long enough to let go before the wheel and handle reached the end. Pinkscrubs' stupid ass little brother kept hangin on and the wheel bottomed out on the lower anchor and sent him flipping off into the opposite bank from 30 feet up or so lololololol
Also, what's gonna be your landing spot? A 40 foot drop with a wheel like we had will kill a ngr. Shit gets you going fast in about 3 seconds