If you were designing a planet atmosphere for 10 billion people, what percent co2 would you target ? Mars and Venus and prelife earth had very high levels of c

If you were designing a planet atmosphere for 10 billion people, what percent co2 would you target ? Mars and Venus and prelife earth had very high levels of co2. I wonder what percentage of the mesozoic co2 is sequestered in oil and gas and what the total quantity is based on that calculation.