My building is 30' X 48' and the floor is PERFECTLY FLAT. I asked for it to be that way.
All you have to do is to allow for your slope when building shelving or benches - or doing anything which requires you to pull a measurement off the floor...
It'd be a lot cheaper for you to just invest in a self leveling rotary level. It's a good tool to have anyway.
Something like this:
http://www.amazon.com/Johnson-Level-...r+level+rotary