The Sketch uses geometric compounding. Each scenario runs a band of three growth paths — conservative, baseline, and upside — which form your Floor, middle projection, and Ceiling. Optimistic: 2.0–4.0–6.5% real (markets performing well above average). Historical: 1.5–3.5–5.5% real (anchored to long-run averages). Stress: 0.5–1.5–3.5% real (compressed returns, heavy downside guardrails). Inflation: Real strips inflation out — all values in today's dollars. Nominal adds 3.0%/yr inflation, showing the raw future dollar amounts.

Plan-Through Age sets how long the money has to last. It's the one input that isn't about how hard your plan works, but how long — and it reshapes everything downstream. A shorter retirement means each dollar covers fewer years, so the same capital safely supports more spending: your Floor, Ceiling, and Datum all rise. A longer retirement spreads the same capital thinner, and all three settle lower. Set it to the age you genuinely want to be covered through — it's an honest assumption, not a dial to make the numbers look better.

Your Floor and Ceiling each use a withdrawal rate tuned to the question they answer: the Floor uses a reduced rate for resilience under a conservative path, the Ceiling uses an elevated rate for capacity under an upside path — and both flex with your retirement length, rising as it shortens and easing as it lengthens. Your Datum is different in kind: it always shows the capital required to sustain your chosen spending — at the canonical 4% rule for a baseline retirement, adjusting with your Plan-Through Age just as the boundaries do. That way the three boundaries answer three honest questions: "what could break" (Floor), "what could stretch" (Ceiling), and "what would my spending cost as capital" (Datum) — each measured against the length of retirement you're planning for.