If you feel that the FMX is correct as far as acres go, then I would agree with Mav.

Hold the Self-test key to review that the 440 is set in US measurements rather than Metric (Si) or Turf (tu). If the NH3 bar is only one section then the total width of coverage (in inches) should be entered into Boom 1.

The other section widths should be set as 0,  In a way this shouldn't matter as long as the other boom sections switches are in the OFF position but setting them to 0 is a good idea.

Another test would be to check the Area/hour.  With the Master ON and Boom 1 OFF, the Area/hr should show 0 as you move (again assuming a one section rig).  The Area/hr should show a reasonable value when Boom 1 is actually ON and applying.