Position your head to face the center of the screen at the distance that is normal for you and press F12. This will set this as the new default position.
I'm fiddling with the demo (v9.45) and will probably post a Confirmed Solution later.
Did you have to input 5040x1050 manually using the arrow buttons or is the res present in the drop-down list ?
I figured out the problem. X-Plane wasn't "taking" my FoV change. It looked like it was in the GUI but it wasn't getting written to the configuration file. I modified the file by hand and now it's working properly.
I had to set the resolution manually because it's not exposed by the TH2Go automatically.