Cut a rectangle of desired size for half circle, then press in half, placing
the fold towards you...(finger is pointing at fold)

Place the solid line of ruler on the fold.... the "raw edge" is along the
dotted line....(where finger is pointing)

Use 28mm rotary cutter and follow inside curve....Placing hand below
the arc that you are cutting in/on....

Opens up to half circle with seam allowance included on the straight
side...