Routing dadoes or grooves to hold a plywood panel can be a challenge. The problem is it measures a little less than the stated thickness. Whenever I need to rout a perfectly sized dado to match a piece of plywood, I turn to a pair of guides like those shown in the photo below.
The guides consist of a hardboard base with a fence on top. I use the router and a 1/2″ straight bit to trim the base of the guide.
Once you've made the guides, using them is straightforward. You can use a combination square to set one of the guides square to the edge of the workpiece. Then place a plywood spacer alongside the guide and clamp the second guide against the spacer.
Now, you can rout along one edge guide then turn the router around and make a return pass to complete the dado.
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