You’ll want to leave the empty space as “free space” (it will get formatted during the Ubuntu installation).
Just open Disk Utility in OSX, and resize your existing OSX partition, so that there’s some free space for Ubuntu. It continues to simplify the installation process, and seems quite stable so far.
I did a fresh install of 17.10 on my Macbook Pro Retina mid-2012 (rMBP 10,1). I skipped 17.04, since it didn’t seem to add much that I was excited about, but 17.10 has switched from Unity back to Gnome3, so I wanted to give it a try.
