The Exterior Backup for V-Grooves controls where the Blade Plungesinto the Matboard (not the Fallout) on the Exterior Cuts of the Outside Cut of the V-Groove. The Exterior Cut is where the Blade Plunges/Retracts from the Matboard (not the Fallout).
If the Exterior Backup on the Outside Cut of the V-Groove is cut Too Long, Decrease the Exterior Backup.
If the Exterior Backup on the Outside Cut of the V-Groove is cut Too Short, Increase the Exterior Backup.