Work on the back edge of the side pieces, take off the width of the middle shelf by lining it up at the end of the boards making sure it is flush before you make a mark. Measure from the opposite end to the mark, Paul’s was 21 ¼” (540mm) then halve this, 10 ⅝” (270mm ), to find the centre. Use the middle shelf piece and line this up with the centre mark, mark the width of this across both the faces of the side pieces.
Square these two lines across the larger face to define the sides of the stopped housing dado, these lines do not need to reach the other side as it is a stopped joint.
The stopped housing dado is the width of the sides minus the thickness of the shelf so to mark the length of the stopped housing dado, use the shelf thickness as a guide. Repeat on the other side piece.
Mark out the waste wood.
Set your marking gauge to ¼” (6mm) and rundown the side of the housing dado.