Argentum Pension calculator.

[RetireJapan]

Entirely depends on your lifestyle, but my impression is that 3m a year would be enough for my wife and me to live a life similar to the one we have now, provided we don't travel much or spend money on family.

Pensions will provide the bulk of that (for us, maybe as much as 4m a year from 2xnenkin + UK state pension), and then some savings and investments to deal with inflation and discretionary spending.

You might be overthinking this a bit.