A system for locating the midpoint of an arc of a bull-nose while masking surfaces adjacent to the bull-nose corner between a pair of flat walls is disclosed. One example of the system uses a flexible planar sheet that has a lower surface having a low-tack adhesive and includes a semicircular edge that is used to mask trim next to the bull-nose corner. Another example uses a tool that includes a semicircular edge that commences tangentially from a straight edge and includes indicia along the concave arc. The arc portion of the shape is positioned against the bull-nose while the straight edge portion the shape is positioned against one of the planar walls so that the midpoint of the arc may be read from the indicia along the concave arc.