Keyboard short-cut to switch to a specific workspace using Ctrl-[1-8] on numeric keypad

I have 8 workspaces in use throughout the day arranged as 2 rows x 4 columns.

Workspaces are numbered logically from 1-4 for the top row left-right, then 5-8 for the next row.

I would like to be able to switch to a workspace using the keyboard combination Ctrl-num where [num] corresponds to number [1-8] on my numeric keypad.

Gnome/Compiz already allows me to switch workspaces via Ctrl-Alt-up/down/left/right or at least that is how I have configured it.

Finding a way to make the mapping and behavior described above however is still confounding me.

Under Preferences -> Keyboard Shortcuts you can map up to 12 workspaces. You can find needed entries under Window Management -> Switch to workspace #.