What is the optimum level of CO2?
Depends on the quality of life you want.
historically global temperature and human society can be correlated (though this is not proof of causation, it does need to be considered).
warmer temperatures (which leads to more C02, not the other way around) in general means better life for everything, plants animals and of course humans, cooler temps show terrible things, like the black plague and the "dark ages".
now taking an overly myopic view and looking only at one small component of this, "c02" is not a very good way to understand a hugely complex system.. but the above should suffice as an answer as long as this limitation is kept in mind.
From what I read current plant growth stops 150-200 PPM and peaks 1500-2000. Greenhouse operators routinely supplement/elevate CO2 levels to improve plant growth. Furthermore CO2 had been at least as high at 7000PPM and the Earth didn't burn to a crisp in runaway heating. In fact those high levels correlate with luscious plant growth.
It is being stated throughout the interweb and repeated here a rise to 400PPM portends global calamity.
Therefore I asked the question what is the optimal level of CO2?