Starting with current assumption
Your current assumption is to manage your monthly expenses within ₹ during your retirement. In this case, you will need to save ₹NaN/month for your retirement.
In case you need to spend less
Instead of considering your monthly expenses as ₹ , if you manage your lifestyle in a way where you restrict monthly expenses as ₹ , the monthly savings required to reach the target amount will go down to ₹NaN.