Retirement planning has two halves. Until you retire, savings grow. After, you spend from the corpus while it keeps earning - but your expenses keep rising with inflation. The corpus needed is the amount that lasts exactly until the age you plan for.
Count EPF, PPF, NPS and funds set aside for retirement as 'already saved'. If you will have a pension or rent, take it off the monthly expenses.
How it is worked out
Corpus = E × [1 − (1 + g)−n] / g × (1 + g), where E is the first year's expenses, g the real return ((1 + return) / (1 + inflation) − 1) and n the years in retirement.
Questions
What return should I assume after retirement?
Lower than before - most people move to safer investments. 6-7% with 6% inflation is a cautious starting point.
Why is the corpus so large?
Inflation. At 6%, expenses double about every 12 years, so ₹60,000 a month today is over ₹2.5 lakh a month in 25 years.