Ah, while I was typing, you were editing.
I don't often set up disks for multiboot, or sharing between OSes. And I don't have a test system handy at this instant to give you any more detailed guidance once you "shrink" your existing partition. But just go by the output of an interactive fdisk session in CHS mode, and you should be fine. Before writing out your MBR, double check that you don't have any overlays.