Per diem/Daily allowance is the allowance given to an employee who is on official tour in connection with company's business and/or who is on a non - residential training program, and is nominated by his company. This allowance take care of his food/tea/snacks expenses reasonably well.
Fixation of per diem or Daily allowance depend upon HR Travel policy which varies from company to company. This further varies from Place to Place; that is Per diem/daily allowance applicable for Metro cities are different and the same applicable for non metro cities are different.
Hence you may start doing the exercise of classification of cities in India (like A-Class cities, B-Class cities, C-class cities) and fix DA/Per diem which takes care of reasonable daily expenses of a person.