- Create a backup just in case
- In Macos use the diskutility to shrink the macos install
- Boot the Debian live: https://git.zerfleddert.de/cgi-bin/gitweb.cgi/m1-debian/
- Install gparted:
        apt-get install -y xinit blackbox xterm firefox-esr lightdm
- Startx:

        startx

- Start gparted:

        gparted

- In gparted move the paritions to the end of the macos install and than grow
  your Linux partition.