One conservative answer might be that you should you assume a return from your investments of 5 per cent per year. So say you had pounds 100,000 invested in various places, you could expect at least 5 per cent back from that as income -pounds 5,000.
Work out what income you need, multiply it by 20, then that's the lump sum amount to aim for.
The exact amount you'll get back depends on your investment stance - savings accounts are paying a pitiful amount of interest and gilts (government bonds) have very low yields as well because interest rates are low.
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