Before joining the two boards together, slotted holes must be cut into the back-up board for the function of changing the fence and mounting position on the table top with recommendation to the fence on the miter saw. These slots need to be a little broader than the shaft diameter of the lag screws - http://www.recruitingblogs.com/main/search/search?q=lag%20screws you mean to use to mount the fence to the table. Cut a few similarly spaced slots in the back-up board perpendicular (at best angle) to the fence.