Loss rate is simply one minus win-rate, but framing it this way changes the planning. A 40% win rate system loses 60% of the time, which means a 6% chance of five straight losses at any given point and near-certainty of seeing several such runs in a few hundred trades.
The arithmetic is worth doing before you need it: the probability of n consecutive losses is loss rate to the power n. At a 60% loss rate, seven in a row is 0.6^7 = 2.8% per starting position, and across 200 trades you should expect that run to occur. See streak-probability and consecutive-losses.
Knowing the number in advance is what turns a losing streak from evidence of a broken system into a scheduled event. Most abandoned strategies were abandoned inside a statistically ordinary run.
Related: win-rate, streak-probability, consecutive-losses, risk-of-drawdown