All I need to mark scribe lines for the back edge of a countertop is a washer. I put it flat on the counter so that it touches the wall where the gap is widest, and put my pencil tip in its hole. Then I roll the washer along the wall with the pencil tip, leaving a perfect scribe line on the counter. The bigger the gap, the wider the washer has to be.
—Jay Kester, Layton, UT