How you measure the slope with a 48 inch digital measure?
If it's a cruddy level it may be different when reversed.  I have one level that is off by about 1/8 over it's length this way. E.g. if it's level one way, reversing it takes a 1/8&quot; rise on one end to fix.  I use this one or rough work, and for drawing lines on plywood.
Ok the level works.  You need a longer level. Go buy the longest straight 2x4 or 2x6 you can that will fit in the room.  Look down the length of it, or better, run a tight string down it to see that it's straight.  Piggy back your level on this.
A far easier way however is to get a self leveling laser level.Set it up so that it's exactly X inches above the floor at some point.  Now with a tape you can run around and map out your floor.  Mine has a couple of gratings that can be put in the path.  One spreads the beam into about a 45&quot; wide beam.  Set up in one corner and do most of the floor with 2 moves.  (The other makes it spread vertically giving you a perfect plumb line.