Center to center, and the mounts are symmetrical. Based on the mount adaptors that came with my Altrider topcase rack - other adaptors may vary.
The two holes at the front are 8 15/16" apart. The two holes at the back are 1" apart. If you drew a line between the front holes, and a second line between the back holes, the lines are 8 1/2" apart.
Or, when I have done this in the past, here's what I did:
1. Measure the case to find the center-to-center distance between the two lower pins.
2. Install the pins on a piece of scrap plywood.
3. Take a wad of silly putty and squash it into a pancake, and put it on the plywood approximately where the top wedge-shaped mount will go.
4. Put the case on the two pins and press it into the silly putty.
5. Pull the case off and place the wedge into the mount hole on the case, and eyeball where it goes. Then press it into the silly putty in about that spot.
6. Pull the wedge off the silly putty and you can see where the holes are. Drive a pencil through the silly putty to mark the center of the holes on the plywood.
7. Peel the silly putty off the plywood, drill holes.
8. Snap the case to the plywood and confirm it fits.
9. If it does, then you have your template. If it doesn't, then you can drill a new pair of holes for the pins, and mark the holes for the wedge again, until you get it right.
Getting the Givi mount in the right spot is the hardest part.