I have a discontinued Dado Wiz, which is for making exact width dados using a straight edge clamp as a guide. You insert a sample of the material and that is exactly the width you get. The router rides on top of it and you use a 3/8ths or half inch spiral bit. If you want to put glue in the dado, you need to add a couple of sheets of paper to the sample piece--it's that accurate. Too bad it failed as a product. Someone is making a version of it, but I don't think it's as complete as the original. Looks like it went out of business about 2013.
Today, a good shop made exact fit dado jig and a bearing guided mortising bit are probably your best bit although there are a couple of commercial jigs still out there.
The more I do, the less I accomplish.