Don't hit your profit target and then lose the payout because one day was too big.
The rule: to withdraw, your single biggest winning day can't be more than a set share of your total profit (Topstep ~50%, Apex ~30%). It stops "one lucky day" accounts from passing.
Why it burns people: they hit the dollar target, request a payout — and get denied because one day was 60–70% of the total. This tool tells you, live, how much more you can safely make today, and how much more total you need to make a too-big day compliant.