I don't know which TrackIR you have but with my TrackIR I don't have a hotkey that allows to define a different position as centre.

I have following Hotkeys:
F7: Precision moving (basically reduces the gradients of the curves)
F8: Profile (whatever this is. But having tried it it does not work as blackdog described)
F9: Pause button
F12: Re-centre (back to initial centre)

I don't have further possibilities to edit hotkeys with the TrackIR software (I use TrackIR 5 software with TrackIR4 hardware)

These are default settings.

Maybe what blackdog describes works with TrackIR5. This I cannot judge. I have TrackIR4 like so many others and it does not work.

EDIT: No, Manu, also pressing F12 in the way as blackdog described it, does not work. It just puts me back to the default centre position.

And even if it did work: everytime I would look around I would have to repeat this manoeuvre as every looking around slightly shifts the centre to some arbitrary position. So definitely not a good solution.

I however support blackdog that the Ju88 gun sight should be fixed. They may fix the other gunsights at the same time. It should not need much programming.