A front yard of not less than 30 feet shall be provided for every building on a lot for which a setback is not hereinafter provided for. Every building shall be set back from the center line of each adjoining street or area reserved for highway purposes, or any extension or separated portion thereof hereafter established, not less than the street setback specified upon the Official Zoning Map, except that the following reductions may be made for a corner lot from the setbacks specified on the Official Zoning Map: five feet when width of lot is more than 80 feet but less than 90 feet; 10 feet when width of lot is more than 70 feet but less than 80 feet; 15 feet when width of lot is 70 feet or less; provided, however, that no setbacks will be less than 15 feet plus 1/2 the width of the abutting road or area reserved for highway purposes.