Hello,
I am interested in these kind of problems.
Maybe the difference is between the RAS (origin = scanner origin) and TkRegRAS (origin = volume center) coordinates.
I encountered this problem marking electrodes as pointsets in FreeView. These are saved using the RAS coordinates.
The conversion code you propose is only valid for the TkRegRAS coordinates. This problem usually produces error of about 1 cm.
Regards,
Clément