After the first notch has been cut in the backer board, insert the spacer block through the notch. It should fit snugly without raising the backer board off of the surface of the table saw.
Slide the spacer block so that the end is two inches past the front face of the backer board, and make a pencil mark on the spacer block against the back side of the backer board. Cut the spacer block at this cut line. Retain both halves of the piece that you cut, as you'll need both pieces.
Next, remove the miter gauge from the table saw and turn it upside-down, exposing the bottom side of the backer board. Place a small amount of glue into the notch and position the spacer block in the notch so that the cut you just made is flush with the back side. Pre-drill and countersink a screw attaching this spacer block to the backer board as shown in the image above.