In all Residential and Commercial Zoning Districts, where lots having 50% or more of the total frontage on one side of a street between intersections (that is, in one "block") are developed with buildings and the front setback of those lots do not differ by more than ten feet, the minimum required front setback on that block shall be the average of the existing front setbacks. However, in any such built-up area, no front setback of less than 15 feet shall be permitted, nor shall any front setback of greater than 50 feet be required.