It's complex. :(

This is from the FSX SDK:

reference_datum_position
Offset (in feet) of the aircraft's reference datum from the standard Flight Simulator center point, which is on the centerline chord aft of the leading edge. 
By setting the Reference Datum Position, actual aircraft loading data can be used directly according to the aircraft's manufacturer.
If not specified, the default is 0,0,0.
Aircreation582SL( reference_datum_position= 0.000, 0.000, 0.000 )
Boeing 747-400( reference_datum_position = 83.5, 0, 0 )
Beech Baron 58( reference_datum_position = 6.96, 0, 0 )

Acceleration FA-18: reference_datum_position     =   29.00,  0, 0    // (feet) distance from FlightSim Reference position, which is: (1/4 chord, centerline, waterline)

So in the Acceleration FA-18, the standard Flight Simulator center point is 29 feet forward of, but neither left/right nor above/below the datum_reference_point.

That should really boggle your mind! It takes a bit to figure it out in your head, but that's just how it is.

[edit]I find it harder to explain it to you, than I do to figure it out in my head! :([/edit]