Just a quick tip on how to shrink an existing CoreStore volume and create a new volume inside the free space. Since the gui Disk Utility won’t offer this feature you have to jump the shell again.
diskutil corestorage list diskutil corestorage resizeVolume 953ED8ED-4373-4A2E-AA0B-747D3EEC1FDE 710G diskutil corestorage createvolume 953ED8ED-4373-4A2E-AA0B-747D3EEC1FDE jhfs+ cleanOsInstall 45G
At first you have to grab the UUID of the volume that should be shrinked (in my case it is 953ED8ED-4373-4A2E-AA0B-747D3EEC1FDE).
Next step is the actual shrinking, 710G is the new target size. With another quick look at
diskutil corestorage list you can verify how big the old volume is and how much space is availabe inside the logical volume family.
Next and final step, create the new Volume inside the LVF with a journaled HFS+ file system, named cleanOsInstall and 45G space.
BTW, make sure to have a backup of the whole drive ;)