That's just it. The ARF doesn't provide any of that centerline or datum line information for anyone to verify where anything is. The instructions don't even mention checking any of the incidences as they are already designed in and set with the completed assemblies. If there was a set of plans then you could see what internal structure is the centerline. The only thing I can do is block it up and choose a surface (in this case the lower wing) as a starting point.

Jaybird