I recommend following this tutorial. In addition it was running a little slow so I found a cheap box of M Snow on Craigslist and added it on to the bottom.
The rail is 18 inches high and I have plenty of speed for it. I'm also pushing off the top railings of my setup like a slingshot.
https://www.newschoolers.com/forum/thread/471848/TUTORIAL--How-to-build-a-summer-setup
There's a lot of stuff on Newschoolers "build a jib", but you need to search it separately. In addition, just start building because almost everything works and you can always make modifications.
[video]1094756[/video]
Hope this helps! (DM with any other questions)