Per diem/daily allowance is the allowance given to an employee who is on an official tour in connection with the company's business and/or who is on a non-residential training program and is nominated by his company. This allowance takes care of his food/tea/snacks expenses reasonably well.
The fixation of per diem or daily allowance depends upon the HR Travel policy, which varies from company to company. This further varies from place to place; per diem/daily allowance applicable for metro cities is different from that applicable for non-metro cities.
Hence, you may start by classifying 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.