This simple project has been on my list for a while. 

The extensions are made from drill rod. They are slotted, drilled and tapped 2-56 and held on with set screws.

The setting gage has holes spaced at 1 inch and 10mm.

To use set the pointers in the gage zero the calipers. Your measurement will be off the distance of the setting gage.

Its accurate within about 1 thousandth of an inch for unthreaded holes. Threaded holes show a little more error due to not being  completely symmetrical.