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Do you know how many concrete garage floors are NOT level?
Lot's of 'em."
Very true. Most are pitched for water runoff. When I visited the The Racer's Group shop in Mooresville, NC, they had special areas in the shop where they did suspension set-up. Those areas had a super-hard, poured pad of epoxy held in place by a small rubber dam all around. The whole thing was maybe a 9' X 16' rectangle and maybe 2" deep. Pour the stuff in and it settles into a perfectly flat, level surface and then cures and stays that way because of the dam around it. Everybody in racing doing suspension setup does it on a pad like that.
For the rest of us, just plan on a non-level floor and make do.