A good ACoS is the one you can handle without losing money. This is also called the “breakeven ACoS”. The calculation for a breakeven ACoS is very simple. Your target ACoS should equal your profit margins because then you know you’re not losing money on advertising. So, if your product’s profit margin is 35% then that is the maximum ACoS you can afford without losing money. Some sellers who are in the middle of a product launch process will tolerate much higher ACoS for a long time, in order to help their product rank better organically on Amazon.