Spectec SDV-841/842 - Manual (Page 21)

Calibrating the Touch Screen
You do not need to recalibrate your pen when switching between virtual
display configurations. NVD automatically adjusts the pen physical location to
virtual display coordinates. The only consideration when calibrating is that
CALIBRATION CAN ONLY BE PERFORMED WHEN NVD IS DISABLED. If
you attempt to calibrate in any other mode, you will receive a warning and
calibration will abort.
Additionally, when using compressed configurations it is even more important
that the pen be precisely calibrated since each element on the screen
becomes smaller. Uncompressed configurations are more forgiving of sloppy
calibration.