Hra exemption

Hi All, Section 10(13A) of IT ACT read with rule 2A of IT Rules stated that For Mumbai, Kolkata, Delhi or Chennai, the tax exemption on HRA is 50 percent of the basic salary, while for other cities it is 40 percent of the basic salary... Now my Question is whether there is any terirorial limit or measurment criteria for Metro.. for Ex. whether a Person residing 30 km away form Chennai will be entitle for 50% Exemption or 40 % will be availble to him. Please Suggest Thanks In advance