Paul
Unless your getting the car corner weighted, where they set them for weight distribution and less concerned about heights.
I'd (did on my own car) have the car on flat ground and set the so the front of the car is level and the back of the car is level. Think i measured from the floor to the top of the arches for convenience and made sure they were the same.

Also good to check that (unless you have helper springs) that the srings on the rear are sat properly on the platforms.