A r/AllocateSmartly thread asked something we have wanted to write about for a while. The poster used FireCalc and Early Retirement Now's toolkit during his index-fund years, then moved to momentum-based investing and realized none of the classic calculators can model it. He is right, and the gap matters more than it looks.
What the classic tools do well
Credit first. FireCalc and cFIREsim run your plan through every historical start year since the 1870s, which is exactly the right method: sequence risk lives in the start dates, and Monte Carlo tools that shuffle returns destroy the very autocorrelation that makes bad decades bad. Big ERN's spreadsheet adds CAPE-conditioned withdrawal rules and is probably the deepest free tool in existence. In fact ERN has recently added a simple momentum model to the toolkit (Part 63 of the SWR series), which tells you serious people see the same gap.
But the core assumption everywhere is a fixed allocation: 75/25, 60/40, pick your split, hold it for 30 years. The tools answer "can this static portfolio fund this spending", and that is the question most of their users are asking.
The question they cannot answer
A tactical investor's portfolio is a rule. HAA is 8 assets gated by a canary, in cash or treasuries when momentum breaks down. GEM flips between US equities, international equities, and bonds. The historical NAV of the rule looks nothing like any fixed blend, especially in exactly the years that determine retirement success... 2000-2002, 2008, 2022.
So "what withdrawal rate does my strategy support" needs the calculator to replay the rule itself through history. That is what we built. For every strategy in our catalog we compute the classic Bengen exercise on the strategy's own backtested NAV: every rolling 30-year window, inflation-adjusted withdrawals, and we report the floor (the SAFEMAX-style worst-window rate) plus the full distribution across windows, because a median without a floor is marketing.
What comes out
The headline results are in the original study and the 1974 stagflation extension. The short version: on our 1993-2026 window, classic 60/40 supports a floor near 6.7%, and extending the data back through the 1970s stagflation pulls it down to 4.5%, which is Bengen's number rediscovered from the other direction. Defensive tactical rules like HAA hold visibly higher floors on the same windows, with the honest caveats those posts spell out: shorter data than a 1926-start study, backtested rules, and floors that move when you extend the data (HAA's went from 12.7 to 10.4 when we added the 1970s).
The caveats are the interesting part, actually. A strategy's SWR computed on 34 years of data is an upper bound, not a promise, and we say so in the product. What the comparison is genuinely good for is ranking: the gap between a trend-gated strategy and a static blend, measured with identical methodology on identical windows, is information the classic calculators structurally cannot give you.
How to use both
Our suggestion is a two-tool stack, not a replacement. Use FireCalc or ERN for the full-history, worst-cohort view of your spending plan with a conservative static assumption. Then use the strategy-level SWR numbers to understand what the tactical rule you actually run has historically added or subtracted on top of that baseline. If the plan only works under the optimistic tool, it is not a plan.
Every strategy page in the catalog shows the full drawdown and recovery history behind these numbers, which is the piece of retirement math that CAGR hides. That is where we would start.