Think of it as an L and you are looking down into the port. You want to measure starting at the front edge of the baffle (ie put the tape measure lip over baffle edge and run it towards back of box).Yea the longer port running parallel to your side will end 3.75 from your back wall. your shorter piece will butt to it and be 3.75 away from the back. The side closest to the back should be flush with the end of the longer port and attach inside the longer port. So the short piece will be 6in long (8.625 - 1.875 distance to center of port - .75 thickness of mdf of other port).