Hi,
DA is calculated based on the Whole sale price index which is the basis of calculating Inflation in India. Now, majority of the companies have done away with DA except Govt and Public sector in India. Reason being, DA is suppose to be added to Basic salary while calculating, Terminal benefits like Gratuity, Bonus as per Bonus Act et.
Now, don't ask me how is inflation calculated...joking. If you don't know, let me know.
Employers State Fedration of India (ESFI) will provide the monthly update by 20th of the month on the Inflationary points based on which DA calculations is involved in these companies. Your city will have a ESFI branch. Check it out, if you need. You will have to take membership with them. They are the central / state govt authorised agency for this data.
Regards