Mr. Thompson is wrong in this case. His advice is only correct if the rear spacing is OK. If one side is out 8mm too much AND that is the excess in spacing, then the dropouts have to be centered after moving that stay in. You can't have identical measurements and it not be centered, period. Please just move the one rear triangle in by 8mm, as originally instructed. If you move both 4mm to one side then you will have to move both 4mm toward the center.