X-Plane 12 allows aircraft authors to specify the exact location of the radio altimeter sensor on the aircraft, and also the precise offset where it reads 0.
This is to allow for correct radio altimeter readout with pitch. Ever wondered why the 737 reads “-4” radio altitude when it is sitting on the ground? That’s because the antenna is in front of the wheels, and thus further off the ground when the plane is pitched up, and it’s calibrated to read 0 exactly when the aircraft touches down in landing flare attitude, with the landing gear struts maximally expanded. Once the aircraft settles in, compresses the struts, and derotates to level attitude, the altimeter reads negative!
Updating aircraft
In X-Plane, radio altitude was naively defined as the altitude of the CG of the plane over the underlying terrain, minus the equilibrium height of the plane fully settled on the ground (struts compressed).
In Plane Maker 12, loading an existing plane, the radio altimeter location and calibration offset is defined to be exactly that – at the CG of the plane, offset for the calculated gear equilibrium. So nothing changes initially, and you don’t have to re-calibrate the radio altimeter if you are happy with the readout of X-Plane 11.
But you can go in and define the antenna location on the plane (with respect to aircraft datum, like everything else you attach to the aircraft in Plane Maker) and the calibration offset you need. Once saved, X-Plane will for this aircraft simulate the radio altimeter antenna being mounted at the given location, and change the readout according to pitch attitude.
The datarefs sim/cockpit2/gauges/actuators/radio_altimeter_height_ft_pilot and radio_altimeter_height_ft_copilot will provide the readout needed for the PFDs. Use this rather than the flight model datarefs for a realistic indication that can go negative on the ground.