[Mighty Midget]

I see your point about ambiguosity. What's your thoughts on this:

Define a horizontal plane that doesn't rotate with a horizontal subvector. That vactor will decrease/increase in length and point in different directions as the mast tilts but crucially, it will be on a constant plane. One feature that can be implemented is to let the user define the "altitude" of the horizontal plane relatively to eyesight, and he can then practice orientations "in all altitudes".

The reason I'm a bit cautious about having the disc there (not to mention the full model) is that it is visual "noise", one of the many reasons orientation is so hard to learn.