Your plan has to answer two different questions, and it's easy to mix them up. One is what could the portfolio support? That's the return model. The other is what rule decides how much you actually spend this year? That's a choice you make, and this is where you make it.
The four rules
They form a ladder: the further down you go, the more your spending answers to your portfolio instead of your plan.
Fixed plan: spend what your plan says, every year, whatever markets do. Your spending is steady and the portfolio takes all the strain. This is the default, and the honest baseline: whatever the other three buy you, they buy it against this.
Guardrail: your plan, nudged. When your withdrawal rate climbs into the danger zone the flexible slice is trimmed a step; when it falls into the safe zone it's raised a step. Smaller swings than the two balance rules below, and correspondingly less protection.
VPW (Variable Percentage Withdrawal, from the Bogleheads community): each year, work out what your remaining balance would support if it were spread evenly over the years to your terminal age, and spend that. The percentage rises as you age, because there are fewer years left to cover.
% of balance: spend a fixed percentage of whatever you have. The simplest adaptive rule, included as an honest comparison: it never rises with age, so it tends to under-spend early money and leave a large unspent balance behind.
The trade nobody should hide from you
An adaptive rule is very hard to run out of money with. That isn't magic. It's arithmetic. If you always spend a fraction of what's left, there's always something left. The rule cannot fail; your spending absorbs the failure instead.
So a strategy comparison that only shows you success rates is worse than useless. It will always crown whichever rule cuts your spending hardest. Read both numbers together:
- the chance of success, and
- what you'd actually be living on in a bad decade (the app reports the leanest year on the 10th-percentile path, and how far below plan spending ran).
A plan that "succeeds" at $19,000 a year is not a plan that succeeded.
How VPW's percentage is set
It's a loan amortization run backwards: spread the balance over the years remaining to your terminal age at the expected return of your stock/bond mix. The app uses the same assumptions as the published Bogleheads table: 5.0% real for stocks and 1.9% real for bonds. A percentage you look up there therefore matches what the app uses. At 65 with a 60/40 mix that's 5.0%; by 80 it's 6.9%.
The terminal age defaults to 100. That's deliberately past most life expectancies, because outliving your money is the risk that matters. Setting it lower means spending faster and arriving at zero sooner. It must stay above the plan's horizon (Assumptions → plan to age). At or below it, the schedule spends the whole balance while plan years remain: the taxes and Medicare due after that final 100% draw have nothing left to come from, so every path grades as failed and the success stats read zero. That's the amortization doing what you asked, not a market outcome, and the panel warns when your terminal age does this.
Where your plan already describes an allocation (Flat or Glide), the stock/bond mix is read from it age by age, so a glide that de-risks over time lowers the withdrawal percentage automatically. On the Blended return model there are no asset weights to read, so you name the mix yourself.
Guaranteed income and the essential floor
Social Security and pensions sit underneath the rule, not inside it. VPW sets what comes out of the portfolio; your guaranteed income is added on top. That's how the Bogleheads worksheet treats it, and it's why the strategy works best alongside a solid base of guaranteed income.
Spending also never falls below the essentials you've marked fixed: Housing, Medical/Medicare, and the non-flexible share of Recurring. That floor is real protection, and it's also the honest failure mode: if the rule's number drops under your essentials, you spend the essentials anyway, and the plan can run short after all. That's not a flaw in the model. It's what running out of flexibility actually looks like.
You set that floor with Flexible share of everyday spending %, which sits directly under the picker because three of the four rules need it. Say 60% and the other 40% of your Recurring budget is essential: the guardrail may trim the 60%, and VPW and % of balance can never take you below the 40%. It is one number, doing the same job from two directions: the part of your budget you could actually give up.
At 100% none of your Recurring budget is protected. Housing and Medical/Medicare stay fixed regardless, but everything else can be cut, and a bad enough run can take a balance rule's spending down to those costs alone. That's a legitimate setting; just make it a choice rather than a default you never saw.
Two things the app does differently
Taxes come out on top. Here your spending number is what you spend, and the tax bill is drawn from the portfolio separately. The Bogleheads worksheet pays tax out of the withdrawal itself. So a tax-deferred-heavy plan draws more than the raw percentage suggests. That is realistic and visible in the results.
Cash counts as bonds. The published table has only stocks and bonds; if your plan holds a cash sleeve, it's priced at the bond return.