Hi, Mike's suggestion is probably your best bet, and about the easiest to manage. Make certain the edges are straight, and that the fixed piece is perfectly square to the side rails. I used to use a simple square for this, but they are not always exact 90 degrees. Buy a set of engineers squares when you want exact 90s, or a really high quality speed square for 90 and 45s.
You will place a piece of the material you are going to insert into the dado to set the width, but I found I needed to add a piece or two of paper in as well to allow for glue.
If you are gluing an edge piece of ply, I'd glue that edge first, then after a minute to let it soak in, I'd glue the dado and insert the piece. You'll only have a few seconds to position the piece to the edges, and be sure you have it set square to the piece with the Dado--that's what those clamp on corner braces are for (see pix).
Once that glue sets, what you set up is no longer adjustable.
The more I do, the less I accomplish.
Last edited by DesertRatTom; 05-03-2017 at 12:32 PM.