It's a better start than simple compounding interest calculations :-)
To approach this from another side, one can buy an annuity (which provides a stream of income for the rest of your life). You need to save as much as is needed to buy such an annuity and then you're good (mostly). However I understand that these annuities are not... attractively priced, especially if you want one which adjusts your income stream for inflation.
That is also my understanding, and I doubt the annuity market has the properties required to make its prices reflect any sort of reality.
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