DIY adjustable height garden bed [Table of contents below]
Step 5. Mark the other side
Turn it all over so the boards are sitting on a couple of runners (the same thickness as a board) fitted into the grooves.
This will keep all the boards in line while you take off the clamps from the underside and re clamp on the top side.
Be sure to position the clamps away from the cut-out area to allow working space.
Square up (with a pencil and square) from the grooves each side, and from those (squared up lines) mark the cut-out area across the edge of the boards.