Don't recall the numbers but it is not measured at the wheel well. As I recall it is the difference between the center of the torsion bars and the center of the wheel. The front is easy to adjust, the rear not so much.
Here is one post on 912 Help citing Clymer manual saying rear difference is 12mm. Search "factory ride height" on that board and see what you can find.
Determining height for rear suspension
Read all the posts in the above thread.
Also, shocks will not affect ride height, only the torsion bars.