In the case of a shopping center in applicable districts, one directory sign may be permitted provided it shall not exceed two hundred square feet in area. For stores within a shopping center, one sign attached to the building not to exceed thirty-five square feet, except by use permit, shall be allowed. In the case of a use permit, signs shall be permitted to have an area not to exceed three square feet for each one foot of frontage, provided that no business establishment shall be allowed a sign in excess of two hundred square feet, and; provided further, that the area permitted may be divided into not more than six single-faced or double-faced signs. Said formula shall apply to each street frontage;