Anyway to shrink a partition in Vista beyond 50%?

OK - so I have a 200 gig hard drive that came as one partition with Vista installed. I'd like to shrink the vista partition to 40GB, and create a 20GB partion to install linux on it, and then make the remainder a data folder to share files between the two operating systems. When I go to the GUI in disk management in vista - it'll only let me shrink the volume by half... I assume I can just run it again to shrink it in half again?