If it's me, i will do a
- 20GB for windows and other add-ons
- 100GB for program files and applications
- 30GB for backup (E.g. setup files, multimedia files or any other files you wanna back up).
I do it this way because it will be more convenient if you intend to reformat one day or rather install another OS without touching the backup partition. Of course, you can choose to take away the backup partition and do all backup on cd/dvd/external disk. It's all your preference. For me, i do both.