Hello Darius,
Yes your backup strategy seems good enough.
You can have 4/5 tapes for differential weekly backups, and you can rotate these alternate weeks.
Secondly you can run full backup every Sunday, and hence have 4 tapes for full weekly backups per month, and one Full Backup at the end of the month.
You can configure your tapes to have Overwrite and Append Periods accordingly.
These technotes should help you further.
The basics of Advanced Device and Media Management for Backup Exec for Windows Servers
http://support.veritas.com/docs/192265
An explanation of the "Overwrite Protection Period" and the "Append Period"
http://support.veritas.com/docs/237374
Hope this helps.
NOTE : If we do not receive your reply within two business days, this post would be marked ‘assumed answered’ and would be moved to ‘answered questions’ pool.
Additional Information :
For information on the recent VERITAS Backup Exec security vulnerabilities, including links to the downloads for the necessary hotfixes, please refer to the following document:
Patch summary for Security Advisories VX05-001, VX05-002, VX05-003, VX05-005, VX05-006, VX05-007
http://seer.support.veritas.com/docs/277429.htm