Hi Art, All of these are good suggestions, but not the easy way. My way is to take a piece of scrap and clamp a strait edge to it and make a cut. Then measure the distance from the strait edge to the edge of the dado you just cut in the scrap. From that point on all you need to do is mark one edge of the cut plus that measurement, and clamp your strait edge in place. You do need to make sure you always keep the router base turned the exact same way toward strait edge. Just mark the router with a Sharpie, and write the measurement down to remind yourself.
Where there's a Willis there's a way!