The breakeven ROAS, or break-even point, is the ROAS level below which a campaign loses money: 1 / margin rate. With a 40% margin on revenue, a campaign must generate at least 2.5 euros of revenue per euro spent. ROI is computed on margin: (attributed revenue x margin rate - spend) / spend.
Example
Breakeven ROAS = 1 / 0.40 = 2.5