Will you have rent / mortgage to pay, and will you receive a full state pension? Both of which have a huge bearing on how “good” your private pension is.
Also, do you have defined contribution pension only, a pot of money to invest, not a pot of money?
Assuming no mortgage/rent, and a full state pension, I think the minimum I would want coming in would be £1500 a month after tax, which means £20K gross (assuming you’re not taking about early retirement, so you have state pension and no NI to pay.
That means I’d need £11K. Assuming a safe drawdown rate of 4%, a pot of £275K would provide for life, without touching the capital. In reality, I wouldn’t want to leave £275K so I would be happy to deplete. Tricky as of course you don’t know how long you’ll live! Or if you’ll need special care.
Of course, you may have a partner which reduces outgoings, but I would personally do my calculations assuming that I’m on my own for my full retirement.
If I had equity in my home, and downsizing was a realistic option, I’d be happier to drawdown more than 4%, knowing that I had an asset to call upon if needed. So that’s another impact on pot size needed - other assets.
Assuming growth of pot only keeps level with inflation, and selling my home only realistic if going into care, I think I’d want 25 years (to age 92) of £11K - which is also £275K. That’s my worst case scenario of zero investment growth.