The V-Groove Interior Backup controls where the Blade Plunges into the Fallout (not the Matboard) on the Interior cuts on the Inside Cut of the V-Groove.
The Interior Cut is where the Blade Plunges or Retracts from the Fallout (not the Matboard).
If the Interior Backup on the Inside Cut of the V-Groove is cut Too Long, Decrease the Interior Backup.
If the Interior Backup on the Inside Cut of the V-Groove is cut Too Short, Increase the Interior Backup.