Yes the Datum or center line as you call it can be an arbitrary thing. It can be anywhere on the airframe and need not follow any particular line. From what I've read many times it is the base of the canopy or the top of the fuse where the canopy sets. If push comes to shove, set the wing at "zero" and see where the engine and stab wind up. I would invest in a incidence meter. The meter will save you HOURS in setup time.

Ken

EDIT; Try the top of the fuse thing. I have a "bubble level" app on my smart phone that seems to work quite well. In addition to the bubble it gives a digital reading of the level in number format.