Getting dadoes cut to the exact right width is the problem, and here’s the solution. Or at least the solution that I prefer.

Given that often the stock we use (plywood, typically) is often not a standard thickness, a special size router bit is needed to cut dadoes in one pass. But you can use a smaller bit and cut it in two passes. The most popular way is to make a jig to guide the router. These tend to be large, and also limited in the length you can cut.

A better way is to use a single straightedge guide and set that to make the first cut on one side of the dado, then use a spacer strip to cut the rest. The spacer strip is sized to be exactly the difference between the dado width and the bit size. Example: if using a 1/2″ bit for a 11/16″ dado, the strip would be 3/16″ thick. I cut these on the table saw from scrap and sneak up on a perfect fit with a test piece.

I show this simple technique in action in this video: