I have a number of installs on here and use data partitions so that all installs can access some common data.
The entry I am using in /etc/fstab for one such partition is;
#Entry for /dev/sda9 :
UUID=58e93156-4981-484c-8047-db53541eaff3 /media/Common ext4 defaults 0 0
I recommend using;
to get the uuid as you have all the info you need there and can copy/paste a lot of it direct or with littel editing.
If you use chroot to access other installs those should mount in /mnt.
All auto mounted partitions and so forth are usually mounted in /media.
In the example above we have the mount point as /media/Common.
To create the mount point is simple no matter where you want to mount the stuff. For my example of the fstab file you would need to run;
sudo mkdir /media/Common