Microadjust with masking tape
When you need a dado that is slightly wider than an available router bit, build up several layers of 2" masking-tape strips on the edge of your router base. You can microadjust a test cut by adding or subtracting layers of masking tape, taking care to place the tape flush with the bottom of the router base.
Make an initial cut with an untaped edge of your router base against the straightedge. Then, rotate the router so the tape runs against the straightedge, and cut in the opposite direction for the second cut. The tape pushes the bit outward, widening the dado ever so slightly.
—Charles Thompson, Washington, Iowa