Custom backup routine – space issues

I have been working on a custom backup routine for a notebook and a netbook; ref. 25851, Custom backup routine. Basically I want to create a simple way to back-up two mobile devices to a USB drive. Here I would like to focus on utilizing space on the notebook and netbook, and the implications this has for a backup/restore strategy. How to organize space on the backup storage device has already been discussed. Included in the discussion are how to assign a meaningful name to a backup file, the value of partitioning the USB drive versus having a more flexible environment, and space requirements for storing multiple Full backups.

Hippodrome
Both the netbook and notebook have one user, and it’s the same person. The operating system is Windows 7 Ultimate edition, 32-BIT. TIH 2010 Netbook edition is used on the netbook; TIH 2011 on the notebook. The netbook’s hard drive capacity is 298.09 GB NTFS format. The notebook has NTFS, 149.05 GB. The target backup device is a Seagate, FreeAgent, USB hard drive, NTFS formatted capacity 465.76 GB.

Space Issues
Should a logical partition be created to separate data from executable programs? If this approach were adopted a Full backup of the System partition could be made while using a different backup mode to backup data. Does this justify the additional complexity?

Data is being categorized by function i.e. My Documents, My Music, My Pictures, My Videos, and My Other. File types are similar within a category and file sizes differ considerably by file type. Therefore a change in mix of activities can affect data storage space requirements, and consequently backup storage space requirements. I am off to see what the data types and file sizes are now, and consider the impact on storage space from changes that are imminent.