There problem with that, first without the bike there, put board and level on ground. Make sure that ground or floor is level, it usually isn't. Basement floors are almost always intentionally off level, so that water spills are aimed toward floor drains. Garage floors will be the same, if there is no drain then the far wall will be higher then the door. Driveways will tilt away from buildings, and toward the street. Parking lots and streets toward storm drains, etc.

Probably a better method is to place the board on the saddle and measure from it to the ground, both at the front and the back. If the measurements are the same, then the saddle is level, if it is not, then it's tilted toward the shorter measurement.