Front yard. There shall be a front yard having a depth of not less than 50 feet, except that where the average setback of existing buildings within the block front is less than 50 feet and forms an established setback, all new buildings constructed within the block shall conform to such established line, provided that no new building shall project closer than 35 feet to the front property line. Where an entire block front is vacant and where the lots are platted now or in the future with a depth of less than 140 feet, there shall be a front yard not less than 35% of the average depth of each lot in such block, but in no event less than 35 feet from the front property line.