Combines are survival games, not profit games. The math is lopsided: to pass a $50K combine you need +$3,000 in profit and you're out at -$2,000 of trailing drawdown. Small profits cumulate — one red day erases weeks.
The trailing drawdown eats everyone
If you're up $1,500 at peak and now down to +$0, your effective DD buffer is $1,500 — half what it was on day one. The peak ratchets up intraday, too, so a morning spike can silently cut your afternoon headroom.
Sizing matters more than entry logic. A strategy with a 60% win rate and a 1:1 risk/reward will still fail the combine if one entry risks 40% of remaining DD headroom. Risk per trade should be small enough that a five-loss streak still leaves you alive.
That's what portfolio.maxSafeQty() does in a Runbook script: it computes the largest qty whose worst-case stop-out stays inside a target fraction (default 25%) of trailing-DD headroom. When headroom is exhausted, it returns zero and your strategy sits on its hands.
Profit target is a trap, not a goal
Most people who fail combines fail them in the final push. They're up $2,200, they need $3,000, they size up to get there fast, they give back $1,500 in an hour. Now they're up $700 — two hours of sideways chop from a breach.
The math says: once you're past ~60% of target, reduce size. The profit target is a finish line you walk across, not sprint at.
Consistency rules: the silent killer
TopStep's consistency rule says no single day can be more than 30% of your total profit. That sounds benign until you realize it triggers after you pass. Traders pass with one $2,500 day and three flat days, get told "you failed consistency," and go back to square one.
The fix: aim for days in the +$200 to +$500 range, stack enough of them, let the profit target come to you.
The sim is the gate
Runbook's combine simulation bootstraps your most recent backtest and replays it forward 10,000 times against the real trailing-DD and profit-target rules. If it says 45% pass and 22% breach on a $50K combine — that's your honest prior before you fund the evaluation.
Use the histogram: if the tail on the left (breach) is fat, you're sizing too big. Reduce size until the breach tail thins out, even if it drops the pass rate. Passing slower is better than breaching faster.