I’m working hard on the fmc. I have chosen to rely only on the X-Plane database, no data files will be specific to the FMC, in order to ensure a clean synchronization between X-Plane and the FMC. Any update of the X-Plane files (including using services like Navigraph or similar) is transmitted automatically to the FMC.
I didn’t imagine how complex the FMC route editor has to be. Just try on the FMCs you known to edit the legs list, containing airway segments, and look at how this is reflected on the route waypoint list, for example. I am really decided to reproduce the real thing as closely as possible, and I’m now taking the full measure of the amount of work needed !