What target ACOS should I set?

Work out your Break-even ACOS first — gross margin after product cost, Amazon fees, freight and returns. If 35 cents of every sales dollar survives, break-even is 35%.

Then decide what you are buying. Profit now: set below break-even, commonly 60–80% of it. Growth: set at or slightly above, accepting thinner margins for volume and rank. Launch: deliberately above, treating the gap as marketing spend.

A target you picked because it sounded reasonable is the most common cause of an account that is either quietly unprofitable or quietly under-invested.