Here, we need to know the Maximum ACoS allowed for a keyword. As a rule of a thumb, if you prefer to be aggressive, you should input your profit margin. If you want to rank fast and increase visibility, get traffic, get volume, get velocity, break even, you sure would want to input there your profit margin.
If on the other hand you want to be more conservative and let’s say you want to make 10% profit for each click, for each sale that happens on PPC, you’d have to figure out your profit margin and do some subtraction to know what to input.
As and example, If my profit margin is 35% and I want to be aggressive, I’ll input 35%.
If my profit margin is 35% and I want to keep 10% profit, then, I’ll input 25%.
In this case, again, I want to be aggressive. My profit margin is 35% so I simply input 35% in there.