A trader with no edge passes a +8 % / -10 % prop-firm evaluation about 56 % of the time
An evaluation with a profit target and a maximum loss is a random walk between two absorbing barriers. For a trader with no edge, +1R and -1R equally likely and no time cap, the pass probability is limit / (target + limit). At a +8 % target and a -10 % maximum loss that is 10/18, or 55.6 %.
I simulated that trader: seed 7, 30,000 paths per cell, one trade a day, p(win) 0.5, 1 % risk, a 365-day cap. The pass share was 55.0 %, within 0.6 points of the closed form. The -5 % daily loss rule stayed inert at that size.
The time cap does the rest at small size. At 1 % risk a 60-day cap passes 30.4 % and a 30-day cap 14.9 %, because the walk needs trades to reach either barrier. Both come from the size and the clock, so a pass on its own is not evidence of edge.
One tail undoes it. A 10 % chance of a -2R gap in place of a clean -1R, with E[R] = -0.10, drops the 1 % risk cell to 22.9 % pass against 77.1 % ruin at 365 days.
The rule set is generic. One trade a day, p(win) 0.5, a 1R bet, one way of modelling the tail. It is not any firm's rulebook, and the arithmetic says nothing about whether any tool changes the number.
