As explained in this video:
Setting your targets depends on type of business you have.
Retention-heavy business: Customer Acquisition Cost targets
Once-off purchase business: Marketing Efficiency or Profit Ratio targets
Retention-heavy business
As retention-heavy business, you usually target based on the Customer Acquisition Costs (CACs), i.e. the investment you're making to initiate a relationship with a customer
Doing so, you want to optimize for profitability & liquidity (how much you want to invest upfront)
Looking at our cohort report shows your CACs and how many months it takes to break-even (based on CM2)
Look at Contribution Margin 2 (CLV)
If your liquidity allows to invest upfront and you don't need to be profitable in the first order, and e.g. want to be break-even after 4 months, the accumulative view in column "4" shows the target CAC you can spend
Once-off purchase business
As mostly once-off purchase business with less strong retention, you usually target based on your Marketing Efficiency Ratio or Marketing Profit Ratio
Look at P&L (past 12 months to get a complete view) to check those values to derive those values from there, taking your CM2 and CM3 margin into account (as explained in the video)
Be aware that the MER is similar to a global ROAS, so it does not directly translate into ad account targets!
Further explanation on the background and how to translate that using Klar attribution in the video