Here's the trick: Use a combination square to locate where to drill the hole in the sheathing and siding. Measure 2⅛ inches—half the hole diameter—from the edge of the rule to the square's leg and make a mark on that leg, as shown. Align this mark with the edge of the drywall, slide the rule to the sheathing, and mark where the corner of the rule and the sheathing meet. Do this in several locations around the hole just to verify the center point.
Now drill a small hole at that point through the sheathing and siding. That hole will show you the center for the bigger hole you'll then cut from outside the house.