The Exterior Backup controls where the Blade Plunges into the Matboard (not the Fallout) on the Exterior cut.
The Exterior Backup field is marked with a Green Arrow on the Configuration screen and the Map illustration.
If the Exterior Backup is cut too Short, it can leave a Hook in the corner.
If the Exterior Backup is cut too Long, Decrease the Exterior Backup.
If the Exterior Backup is cut too Short, Increase the Exterior Backup.