A heuristic from the Trinity Study (1998) suggesting that withdrawing 4% of the initial pot in year one, then adjusting for inflation each year, historically had a high probability of lasting 30 years.

How is real value calculated?

Real value divides the projected nominal pot by (1 + inflation)^years-to-retirement, with inflation assumed at 2.5% per year.

Why doesn't my pot last forever even with growth?

When target income exceeds the growth your pot earns each year (r × pot), withdrawals eat into capital. The closed-form formula determines exactly when the balance hits zero.