My tie ended up slightly above head level so there was not chance of getting caught. I was using the column behind the wall of the barn and it had solid wood up until that point so the wall prevented it from sliding down.
The time I had to do it on a post with out a wall I actually did two things to prevent it from falling down.
I put a small screw eye on the back of the post then took the inner tube, wrapped it around the post and tied it like you do when you start your shoe laces (half a knot I guess) at a point above the screw eye. Then ran the lead rope thru the ends of the inner tube. It never really loosened up enough to slide down but if it had loosened it being above the screw eye (that is on the off side from the horse) would prevent it from sliding down.
I love this set up. It taught her to stand patiently and the few times she decided to fight it it had just enough give but not near enough to let her get loose or anything.