Unless you have a very large compass or a set of trammel points, the string approach is simple and accurate enough for an Adirondack chair. If the pencil is placed in a knotted loop and the other end wrapped around a small pin and held tight, the slight change in radius-length will be negligible. But, after drawing the arc I would backtrack to ensure that the length did not change.
Cut out the template (masonite/hardboard/plywood) and use it wherever called for. |