Hra u/s 10(13a) interpretation for city of residence

Dear All,

As per section 10(13A) 40% or 50% of salary calculated while deducting from HRA given, depending on city (Mumbai, delhi, Calcutta and Madras).

Is there anything from Income tax which say we can give 50% for metro cities, and what is the defination of Metro city to clarify suburban areas of Mumbai viz. thana, navi mumbai included in it?

Thanks 

Amit