The remaining-horizon annuity
Variable percentage withdrawal does not start at the assumed return. Year one of a 40-year plan at 3.5% real is the 40-year annuity factor, which is smaller than 3.5%. As years left fall, the percentage rises, so the rule tries to spend the balance rather than preserve it for an heir. That is the design, not a bug.
Ruin, in the “portfolio hits zero” sense, is not how VPW fails. Income floats. A bad decade cuts the cheque. The risk moved from insolvency to lifestyle. Households that need a floor should not use VPW as the only rule.
Run it
The variable percentage withdrawal calculator is the identity, plus an optional 1,000-path band on last-year spending. Merton is the cousin that also picks a risky share from risk aversion. Both live on the nine-rule hub.
The assumed return
Bogleheads VPW worksheets commonly use something like 3% to 3.5% real for a stock-heavy mix. A higher assumed return spends more now and leaves less later. It is a knob, not a forecast. Changing it on the calculator is the whole point of having the page.