as its drawn up, its ready for a 3/4" board. what i did was to run the router with a 3/4" bit, and ran it with the flat side of the base against the left side board, about an inch into each long board to register where the cut line will be. i would draw a line on the board to be tennoned, and match the line to the left cut line, and then clamp the whole works down tight. then i would start at the end of the board, and work my way back to the cut line. then flip the board over and repeat this for the other side.
needless to say, this is just the basic setup, and will need to be adjusted to the width and thickness of the lumber you are using. all i can say is that it worked for me, and i got nice clean cheek cuts.