You have to do 30x annual expenses with compounding inflation on your expected purchasing power at the start and go with 4-5% inflation per year for 30 years to get your real number. In fiat at least. If you measure in Bitcoin you can probably make a pretty good judgement with that.

You wanna spend 50k per year and retire now let's say. So Total = $50,000 × (1.04³⁰ − 1) / 0.04 Total ≈ $50,000 × 56.085 ≈ $2.80 million If you were 60 and wanted to stop working now and spend 50k in purchase power over the next 30 years, you need 2.8m USD of course.

50k will be the new 25k by the time its time to retire. Lol