Mine was not quite that bad, but that's where I created 4 marked patches that, with the plywood "shims", created 4 places that are flat, so I park the car tires on those 4 places.....I had to take the car and scales over to a friend's garage - with a nice flat floor. My garage floor is like a hyperbolic paraboloid (it ain't flat).
That's the way to do it. But with you, or a proper substitute in the car, I had suggested shimming the pivot bushings, and/or adjusting adjustable end links, so bars are not stressed with driver strapped in.Since there was two of us, I was able to sit in the car as he read the scales. I set all the tire pressures to 35psi. I disconnected the swaybars AND the front & rear shocks. This allowed us to really bounce the car to relieve the spring bind which happens as you crank the perches up and down. After a couple of good bounces you would hear the spring go BOING. Then we'd roll it back on the scales.
Final point about Motion Ratio, the simple leverage IS the motion ratio, and you use the square of the MR in finding the wheel rate for the spring, and for the s-bar.