Setup hard drive partition for both OSX and Windows

I just got an iMac with with a 1 TB hard drive and have installed Windows on it (via Boot Camp). I'd like to add a partition for my files that I can have read/write access from both OS's. What format should I use?

Example:

100 GB Windows, Format: NTFS

100 GB OSX Format: Mac OS Extended Journaled)

800 GB Files <- What format?

EDIT: Turns out it's not trivial to add a third partition. I went ahead and reinstalled Windows and made its partition 900 GB. Thanks for all the help though!