What does “NewModel (stl)” contains? Does it have landmarks that you can touch with a tracked pointer? If yes, then you can collect points in “Marker6” coordinate system and mark the corresponding points on the model (“NewModel” coordinate system) and landmark registration computes NewModelToMarker6Transform.
In general, if you name coordinate systems and transforms consistently then the you it is much easier to see what is the correct solution:
- TrackerToRas
- Marker8ToTracker
- NeedleTipToNeedle
- NeedleModel_Needle
- Marker6ToTracker
- GuideToMarker6
- NewModel_Guide
- CTData_Ras