How To Set No-Parallax Point
Your panoramic head needs to be calibrated precisely so that the camera and lens optical system rotates around the no-parallax (sometimes called nodal) point which virtually eliminates a parallax error from the image geometry and from the final panorama.
Thanks to this setting your photos will overlap precisely, there won't be a shift of close objects on the background which avoids the discontinuous lines error the in the final stitched panoramic photo.
John Houghton's Guide to Finding No-Parallax Point (great guide, scroll down to chapter 5 for ring-based panorama heads such as Nodal Ninja R1/R10/R20).
Entrance Pupil Database (open wiki database)
For setting a no-parallax point and for guessing the necessary length of rails of your panoramic head for a specific combination of your lens and camera you can use the Nodal Ninja database (very old, now deleted, an archive link provided)
Ellaborate description of the no-parallax point problem (hugha.co.uk)