We built a 12' double barrel rail, roughly 20" tall. Our drop in ramp initially consisted of a rectangular tower with an 8' piece of 3/4" ply (supported by 2x4s) acting as the tranny. The tower was roughly 6' tall and unequivocally too short! At those dimensions, it was impossible to slide the entire rail despite every combination imaginable of slippery stuff and water. We added another 2' to the tower and extended the tranny (a second piece of ply) in order to maintain the same angle drop in. This ended up working great.
Recap: A 12', 20" tall rail required an 8' tall (10-11' long) drop in.
I have a 20" tall rail thats 10 ft long and it works fine with a 5 ft drop in with an 8 ft tranny. I also have it set on a downhill so I would suggest 6 ft.. maybe 7 but that gets a little high for just a back yard.
"im 5'9" and i just got my XXL ronin pants and they fit perfect" - PipeCleaner44
hey man i built a backyard setup last year and it didnt work we kept tuning it for three months before it finally worked with soap and water, if u want my drop in ramps dimensions il send um to u, just pm me