You are correct. The tee is placed in the middle of the wall, 12" away.
I don't adjust the tee location or height when working with the wall. It is definitely designed for a low pitch. If the height of the tee is too high the wall is worthless because the bat path wouldn't cross paths with the wall no matter what the swing is like.
The main purpose of the wall is to improve the bat path/direction of the swing.