One other option you might consider is to measure from the center of the bit to the edge of the guide rail. Cut a piece of ply exactly to that width. Lay one edge of this piece to the top edge of the dado location, then snug the guide rail up to that spacer piece. Using a bit smaller than the dado, cut one edge of the dado. Turn the workpiece, then reset the spacer to cut the other side. I'd mark the edges with a knife to increase accuracy. But the best way to do this would be and exact fit dado jig, which you can find plans for online, or search the Forum.