Ken,
After you tweak yours, what do you get for numbers from the timer Y and X when the plane is leveled on the bench upright and inverted? What does the plane look like on the bench upright and inverted if you level the sensor to Y = C, upright and inverted? As you noted, the C value is a value sent from the sensor on a carefully leveled table, then stored. That is the reference value and the reason that Y should equal C.