What a consistency rule costs. A best-day cap is a tax, steepest on the big-day trader.

A consistency rule is easy to underestimate. It sets no new drawdown and moves no profit target; it only says that no single day may make up too large a share of your total profit. On paper that sounds like a formality. In the model it is one of the most expensive clauses an evaluation can add.

For a trader with a real but modest edge, the model's probability of passing peaked at 36.1% with no consistency rule. A 50% best-day cap pulled that peak down to 32.0%, a 40% cap to 26.2%, and a 30% cap to 15.4% — a 20.7-point fall from the strictest cap alone.

The same edge, the same drawdown, the same profit target — the only thing that changed across the four rows below is how much of the total profit one day is allowed to carry. Each tightening of that single clause lowers the best achievable odds.

• No cap · size 1.1%36.1%
• 50% cap · size 0.9%32%
• 40% cap · size 0.8%26.2%
• 30% cap · size 0.7%15.4%

The rest of this article is about two questions that ladder raises: why a rule that adds no drawdown should cost so much, and who among traders pays the most.

The mechanic is precise. When an account would otherwise reach its target, the rule checks its single best day. If that day's profit is larger than the cap's share of the total — more than 40% of total profit under a 40% cap, more than 30% under a 30% cap — the pass is refused. The account cannot bank the result; it has to keep trading to dilute that best day with more ordinary days.

That is where the cost hides. Every extra day spent diluting a good day is another day exposed to the drawdown, on a trailing floor that has already ratcheted up behind the account. The rule does not add a wall of its own — it takes the profit that should have ended the evaluation and forces it back out onto the field, where the existing walls can still catch it. So a lower cap is a stricter rule: the smaller the share one day may hold, the more diluting days it demands, and the more chances the account has to breach on the way.

Under the 30% cap, at the best size, the model's probability of passing collapses to 15.4% — down from 36.1% with no cap at all. And the consistency clause is rarely the named cause of failure: in the model it is the direct reason only about 7.1 times in a hundred. Most of the loss is indirect — forcing an account to keep trading to dilute a big day is what pushes it into the drawdown or the clock instead.

If the cost grew evenly, each ten points of tightening would remove the same amount of the probability of passing. It does not. The four curves below share a trader and an archetype and differ only in the cap; the stricter the cap, the lower and the earlier the peak — and the gap widens as the cap tightens.

The first step is cheap and the last is brutal. Moving from no cap to a 50% cap cost about 4.1 points of peak probability. Tightening to 40% cost about 9.9. Tightening again to 30% cost about 20.7 — the curve does not slide, it collapses. A modest edge has only so much room to keep producing ordinary days without touching the floor; past a point, the rule demands more diluting days than the account can survive.

Notice too that each stricter curve peaks a little to the left. Because the rule punishes concentration, the account is nudged toward smaller, steadier sizing — but smaller sizing runs harder into the other wall, the profit target inside the window. That is the squeeze a consistency rule creates: it makes the big day dangerous and the small day slow at the same time.

A tax is only fair if it falls evenly. This one does not. To see who pays, hold the cap fixed at 40% and run two traders with opposite shapes of edge: a big-day trader who wins less than half the time but wins big when they do, and a grinder who wins often and small. Both are given the same per-trade cost; each is run with and without the cap.

The 40% cap cost the big-day trader more. Its peak probability of passing fell from 50.5% to 39.4% — a 11.1-point drop — while the grinder fell from 33.2% to 25.1%, a 8.1-point drop. Same rule, roughly 3.0 points more taken from the trader whose edge is concentrated.

The reason is mechanical, not moral. The big-day trader's edge is the occasional large day — that is where the profit comes from. A best-day rule points exactly at that day and says it may not count for too much, so the very structure that makes the edge work is the structure the rule penalises. The grinder, whose profit is already spread across many similar days, barely trips the clause. A consistency rule is not neutral about style; it quietly prefers the trader who was going to be steady anyway. That last comparison is in absolute points, though, and the two archetypes differ in the size of their edge as well as its shape; measured as a share of each trader's own odds the two costs land close together — roughly a fifth to a quarter of the peak in both. Read the gap as the direction the clause leans, not a clean coefficient.

None of these numbers is transferable. This study used one synthetic archetype and a small set of illustrative traders; a real evaluation combines a consistency rule with a daily loss limit, a minimum number of trading days and its own drawdown style, and each of those changes the arithmetic. The value here is the shape of the mechanic, not any single coordinate.

Three things do carry across, and they are the point:

• A best-day rule adds no wall of its own, yet it is expensive — it forces good results back onto the field where the existing walls can still catch them.
• The cost is non-linear: tightening the cap removes more of the probability of passing each step, not the same amount.
• It is not style-neutral — it takes the most from the trader whose edge is concentrated in a few large days, and the least from the steady grinder.

Measuring the tax for a specific set of numbers is a computation, not a guess. The PropSurvival simulator runs exactly this sweep on a trader's own win rate, average win and loss, cost and trade frequency against a chosen rule set — a consistency cap included — and reports the same curve shown here. This article models a synthetic archetype and asserts nothing about any firm; for one firm's specific consistency terms, see the consistency-rule guide, which describes that firm's clause rather than quantifying the mechanic. How much to risk per trade covers the sizing trade-off the cap tightens, and which rule breaks first ranks the clauses for a single set of numbers.

What is a consistency or best-day rule?

A clause that limits how much of an evaluation's total profit any single day may contribute. If one best day exceeds the cap's share of total profit, the account cannot pass on that profile — it is forced to keep trading to dilute the best day, taking on more drawdown exposure, or it fails. A lower cap percentage is a stricter rule.

How much does a consistency rule lower the probability of passing?

In this model the tax was steep and non-linear. For an illustrative trader the peak was 36.1% with no cap; a 50% cap cost about 4.1 points, a 40% cap about 9.9, and a 30% cap about 20.7 points, cutting the peak to 15.4%. Each further tightening cost more than the last.

Which traders does a best-day rule hit hardest?

The trader whose edge is concentrated in a few large days. At a 40% cap the big-day archetype's peak fell from 50.5% to 39.4% — a 11.1-point drop — while the grinder fell from 33.2% to 25.1%, a 8.1-point drop. The rule caps exactly what the big-day trader depends on.

Can a trader size around a consistency rule?

Sizing helps at the margin but cannot undo the mechanic. Even at the best size in the model, the 30% cap left the probability of passing at 15.4% — most attempts still failed. The cap does not care how a profit was earned; it only limits how much any one day may contribute.

Is this a claim about a specific firm's consistency rule?

No. Every figure is model-derived for one synthetic archetype and illustrative traders; the study ranks no firm and describes no firm's business. It shows the shape of what a best-day mechanic costs. For one firm's specific terms, see the consistency-rule guide; this study quantifies the mechanic, it does not describe that firm.