Yeah, it's a real pain in the ass. I couldn't find anything to use.
I ended up using my garage. It's sloped in on a hill, and the garage itself is sloped for drainage, so I backed the car out of the garage until it was level, marked the tires so the distance could be duplicated. Then put a level across the fenders, a jack under one corner to level the car side to side. Hang a piece of plywood at the front of the garage as a sighting board, and scale 2.1" drop at 25 feet for the 17'4" I was using. Then take the longest level, figure out the slope of the garage floor, and accommodate for drop.
It worked perfectly, but damn if it didn't take a lot of figuring.