The optimal is whatever it median value was before industrialization. Not that Earth hasn't had life at a wide range of CO2 levels, but current life had a mill

The optimal is whatever it median value was before industrialization. Not that Earth hasn't had life at a wide range of CO2 levels, but current life had a million+ years to get used to mid-200s PPM, whereas we've gone from that to over 400 in a few decades. Ecologically, that's like hitting a brick wall. The rate of change is too fast.