Using a straightedge guide is one of the best ways to rout dadoes and grooves. But trying to remember the offset distance from the outside edge of my router base to the cutting edge of my bit is a pain. Even when I do remember, I always feel a little uneasy, hoping that the cut will be in the right place. A quick and simple way to ensure accurate cuts every time is to attach an alignment board to your straightedge guide.
You now have an exact reference point for all your cuts. Just line up the dado in the alignment board with the marks on your workpiece.