Rout dead-center dadoes
This guide helps you center a straight bit of any width on the centerline of your dado. Start by ripping two strips from 3⁄4 " plywood or medium-density fiberboard, to the dimensions shown in the drawing. With the edges butted tightly together, connect the pieces with hinges. Next, mount in your router a straight bit the same diameter as the thickness of the stock going into the dado. Adjust the bit for the depth of the dado.
Now, mark the center of each dado on your workpiece. Align the flip-up gauge edge with a dado mark, and square up the jig to the workpiece. Then clamp it in place. Flip the gauge up over the fence and use the fence edge to guide the router as you dado-cut.