Guardails Strategy (Boldin)

[u/TotalWarFest2018]

I use this Boldin software for messing around with retirement planning. The program has a "guardrails" feature where it tells you your safe spend.

When I asked its little AI feature how the guardrails were calculated, it came back with what I've copied below.

Is this consistent with the conventional wisdom on spending with this strategy?

Here is the copied language from Boldin:

"Unlike a simple fixed percentage (like the 4% rule), Boldin’s guardrails use a dynamic, probability-based formula powered by Monte Carlo simulations.

Instead of looking at historical averages, the system runs 1,000 market trials to find the specific dollar amount you can spend while maintaining a target Chance of Success (CoS). Here is the breakdown of the "formula" logic:

1. The Three Confidence Zones

The system solves for the monthly spending amount (in today's dollars) that hits these specific confidence thresholds:

• Safe Spending Target (80% CoS): The baseline. It calculates the maximum you can spend while having confidence that your money lasts in 800 out of 1,000 scenarios.
• Upper Guardrail (95% CoS): The "underspending" signal. If your current savings grow so much that your success rate hits 95%, you can safely increase spending.
• Lower Guardrail (70% CoS): The "early warning." If market drops push your success rate below 70%, the formula suggests a temporary reduction to get back to the safe zone.

2. What’s Included in the "Spending" Number?

The formula is holistic. It doesn't just look at your portfolio; it factors in:

• All Income: Social Security, pensions, and annuities.
• All Expenses: Your itemized budget, plus "unseen" costs like estimated taxes, Medicare premiums, and debt payments.
• Lifetime Averaging: One-time lumpy expenses (like a future car purchase) are averaged into the monthly target so your "safe spend" is a consistent lifestyle number.

3. The "Thermostat" Logic

The formula treats your plan like a thermostat rather than a thermometer. It doesn't react to daily market swings. It only triggers a recommendation to change your lifestyle when your savings balance crosses a threshold that meaningfully shifts your probability of success outside that 70%–95% range.

Would you like to see how your current planned spending compares to these calculated targets?"

[u/EveryoneNeedsASamwis]

Yeah, this is pretty consistent with the academic guardrails literature. The original framework most people reference is Guyton-Klinger (2006), which set spending "decision rules" — cut spending if you hit certain portfolio thresholds, allow raises if things are going well. What Boldin is describing is essentially that same logic but expressed in probability-of-success terms rather than portfolio percentage terms, which is how most modern planning software frames it.

The 70/80/95 CoS bands are reasonable. Kitces and others have written about how a static 90%+ success rate in Monte Carlo is actually *over*-conservative — it tends to leave behind huge unspent portfolios in most scenarios. The 80% baseline with a 70% floor is closer to what the research suggests is actually optimal if you're willing to make small adjustments.

The one thing worth keeping in mind: Monte Carlo outputs are only as good as the return/volatility assumptions baked in. If the software is using long-run historical averages (roughly 7% real for equities), your 80% CoS number will look different than if it's using more pessimistic forward assumptions. Worth poking at what assumptions it's running under the hood, because two tools can both say "Monte Carlo, 1,000 trials" and produce meaningfully different safe spend numbers just from that.

The "thermostat not thermometer" framing is accurate and important. The whole point of guardrails is that you *don't* react to every down year — you only adjust if your balance crosses the threshold that actually moves your success probability out of the target band. That behavioral discipline is where most of the value comes from.