Run 9 setups.  Each number below is a setup for 2 measurements (one line)
1.  Measure mirror plates on
2. Remove mirror plates and reinstall.  Install bottom two screws first, then middle then top
3 Remove mirror plates and reinstall.  Install middle two screws first, then bottom and then top
4. Switched top plates and bottom plates.  Install bottom two screws first, then middle then top
5. Measure again no changes
6. Switch upstream and downstream plates.  Bottom and top plates put in proper places. 
7.  Swap back to original positions.
8.  Measure with same setup as 7.