Definitive Tech ProSub 80 Placement

Since the ProSub 80 is ported at it's rear, how far from the wall does the rear of the sub need to be per Def Tech's instructions? I have been offered the above sub from a cousin at a very cheap price, but am going to have to place it within 2 to 3 inches from the rear wall. I am concerned as to the port being so close to the wall. (My cousin has no manual)