The best way to determine how much money you will need in the future is to know what you need right now. Some people think they need their current income. That is not true. You need "cash inflows" to cover your current expenses, taxes, and savings for future needs.

In retirement, you will need cash inflows to cover expenses and taxes only. You may wish to continue saving for your heirs, but that is not a need. Your expenses may change if you change your lifestyle, and your taxes will change depending on whether your cash inflows are from "income" or a return on investment.