Backup Exec could offer an option, where no hard time period is given for the overwrite protection of B2D files, yet they are kept until they have been successfully duplicated to tape.
This should have two advantages:
If everything works well, a backup is done on B2D and the duplicate is triggered by the rule right afterwards. Depending on the amount of data, the duplicate will be done in a couple of minutes or hours and the B2D files can be overwritten. The Backup admin doesn't have to do any calculations and time period safety margin planing.
If things dont work well i.e. all tapes are full, the tape library has a problem => all backups are kept until the problem is fixed and no backup has to be performed again, only because the B2D files have already been deleted.