The formula
ROAS is ad attributed revenue divided by ad spend. It is easy to read and easy to flatter: attribution windows, branded search capture and sales that would have happened anyway all inflate it. It also ignores margin, which is why a healthy looking ROAS can sit on top of a loss making campaign.
Break even ROAS
Divide one by net product margin. At a 25 percent net margin the break even is 4x. At 15 percent it is 6.7x. At 10 percent it is 10x. Industry average ROAS is often cited near 5x, while average fully loaded net margin in many CPG categories sits under 15 percent. At the average, the math does not work, which is why retail media needs to be judged against margin.
What to track alongside it
Use TACoS for total business health and New to Brand for acquisition value. A useful executive ratio is TACoS divided by underlying net margin: above roughly 0.6, the channel is funding Amazon's media business at the expense of its own profitability. The mechanics are in the essay on your Amazon P&L.