There is a way to shrink the partition to less than 50%.
Windows stores system security file in the middle of the partion, you have to switch that off first.

1. Go to Configuration > System > System Security.
2. Click on C: and then Configure
3. Select disable system security.
4. Probably you have to reboot.

Now you should be able to shrink the partition below 50%
Switch system security back on.

Rob