We Monte-Carlo'd a 50K prop eval (profit target vs trailing drawdown) thousands of times at every contract size, feeding it our strategy's real daily P&L. The result surprised us into a two-phase doctrine.
| Size | Pass rate | Median days to pass |
|---|---|---|
| 1 micro | 43.9% | 123 |
| 2 micros | 42.1% | 44 |
| 5 micros | 46.5% | 11 |
Pass probability is flat at every size. An eval is a race between a profit target and a trailing drawdown — multiply your bets and both scale together, so the odds never move. Sizing buys speed, not probability. (Speed has value — evals bill monthly — but know what you're buying.) We also tested clever dynamic sizing against the eval. Same wall: the geometry is scale-invariant. Only a genuinely better strategy improves eval odds. Nothing else does. Anyone selling you an "eval passing" sizing trick is selling you the treadmill.
A funded account with a buffer has no target racing you. There, volatility-scaled sizing — risking a constant number of dollars instead of a constant number of contracts, so quiet days get more size and wild days get less — raised our risk-adjusted return by ~25% and nearly doubled the return-to-drawdown ratio. Same trades. Same signals. Only the when-big-when-small changed. And it passed the full overfitting gauntlet (PBO 0%).
Our rule is literally written down with a formula and a "recomputed Fridays only" clause — because the one guaranteed way to ruin good sizing is resizing after a loss.