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Draw a line on the boards marking out where you want the cut.
On the table saw, find the distance from the outside edge of the table to the edge of the blade.
Make a mark on the joined boards corresponding to that measurement.
Use a framing square (Image 1) to extend the mark all the way across both boards (Image 2).
Place a strip of wood across the boards. It should be placed on the side of the line opposite of where you want to make the cut.
Clamp the strip to the boards.