Max CPC = Break Even CPA / (Click/Order Ratio)
Example: Profit Margin of 30% and Click to Order Ratio of 7
If someone is selling a $10 item with a 30% margin, that means they can spend $3.00 to acquire that sale at target ACoS. If it takes 7 clicks to get a sale on average, the max CPC would be $0.42.
See also: What is the difference between New Bid and Max CPC?