Computation of area of individual signs. The area of a sign face (which is also the sign area of a wall sign or other sign with only one face) shall be computed by means of the smallest square, triangle, rectangle or circle, or combination thereof, that will encompass the extreme limits of the writing, representation, emblem or other display, together with any material or color forming an integral part of the background of the display or used to differentiate the sign from the backdrop or structure against which it is placed, but not including any supporting bracing, or decorative fence or wall when such fence or wall otherwise meets regulations of this chapter and is clearly incidental to the display itself. In computing total "sign area," the area of the faces of all sign displays shall be counted, and any neon tube, string of incandescent lights or similar device shall be considered as having a minimum dimension of three inches.