The return on ad spend at which an ad neither makes nor loses money: 1 divided by your contribution margin before marketing. It is the number that turns a ROAS report into a verdict. A 40% contribution-margin brand breaks even at 2.5x ROAS; drop to 30% margin and the bar rises to 3.33x; at 25% it is 4.0x. Same ad performance, three different verdicts, because the margin, not the ad, decides what 'good' means. The mistake is setting a ROAS target from industry gossip instead of deriving it from your own margin - two founders can run identical campaigns and only one prints money.
Benchmark. Break-even ROAS = 1 / contribution margin (before marketing), then add the profit you actually want on top. Use it to judge each channel's platform ROAS, but manage the business on blended MER, which cannot be inflated by attribution games.