Find the monthly pension your NPS annuity part could pay you.
The portion of your corpus used for the annuity earns a payout rate you specify, converted to a monthly figure.
Monthly pension: (Annuity corpus × annuity rate) ÷ 12
Why is the annuity rate a guess, not a fixed number?
Annuity payout rates are different for each insurer and plan, and also depend on interest rates at the time you buy it. There is no single fixed rate to use ahead of time.