Each hole center is 20mm from the south edge of the keyboard and 78mm from the side edge.
However, having used these for a while, believe that I placed the buttons too far toward the middle of the keyboard. I can see part of the button if I look down while I'm typing, and find myself having to move my hand inward in order to push the button properly.
The holes should therefore be closer to the keyboard sides, ie I would drill the pilot hole at 70mm (or even less) from the side edge of the keyboard, so the buttons are more directly under my hands.
Having said that, I love this mod, it's an absolute MUST. Way better than a footswitch which gets kicked around and forgotten. The palm keys are always there, where you need them.
It's also a much easier mod than any other key addition I tried.