In any R District there shall be a minimum front yard required as stated in the yard requirement for that particular district; however, in no case shall the front yard depth be less than three-fourths of the smallest front yard depth of an existing building in the same block, located within 200 feet of either side lot line and fronting on the same street; however, this regulation shall not be interpreted as to require a greater setback than double the minimum required front yard for the district in which it is located; or to permit a setback which is less than the minimum required front yard for the district in which it is located.