Hello Mark,
The overwrite period of a media starts at the end of each job run, not the beginning. To explain it in a better manner, refer to the following technotes which will guide you as to how you can configure media sets as per your requirement.
Title:An explanation of the "Overwrite Protection Period" and the "Append Period"
http://support.veritas.com/docs/237374
Title: The basics of Advanced Device and Media Management for VERITAS Backup Exec (tm) for Windows Servers
http://support.veritas.com/docs/192265
In addition to the above, the following Technote can help you with regards to creating partitons:
Title:Creating, configuring, and targeting a backup job to a Robotic Library partition in Backup Exec 9.x and 10.0 for Windows Servers
http://support.veritas.com/docs/262055
Regards.