Use a four foot masons or carpenters level on the floor in both the X (tongue to bumper) and Y (drivers side to passenger side) axis. When you use a longer level, you get a better average than you would with a torpedo level.
Now for the fun part. Take your reading at the front floor area then compare it to the rear floor area. When you find that they are different, then you walk outside, slap your sticky levels on, and realize that "close enough" is just fine.
I stuck mine on the front drivers side corner of the 23B. That way I can walk back and check them when I'm setting up alone.
2006 23B Hybrid with 10k round bar WDH
2011 F150 4x4 SCREW Ecoboost, Max Tow, Integrated TBC, 3.73 LS axle, Firestone Ride Rite airbags.