whats the optimal level of greenhouse gas emissions?

Greenhouse gases are given off to the atmosphere by natural processes, like decay of organic matter, volcanic activity, etc. Those gases are taken out of the atmosphere by other natural processes, especially by absorption in the ocean and the growth of plants. A natural balance has developed, an equilibrium condition where the amount put into the atmosphere equals the amount taken out by natural processes.

When additional greenhouse gases are added to the atmosphere the equilibrium shifts and becomes stable at a higher concentration of these gases in the air. That results in a higher temperature of the atmosphere.

If we were creating the world anew we could decide what we would want the concentration to be, what would be ideal. But we can't. Civilization and the plants and animals of the world have adapted to the existing natural conditions. Changes in world climate will change weather patterns, wind and rainfall occurrences, hurricane distributions and severity, sea level elevations - things that will cause economic hardships and redistribution of animal life and extinctions of some.

So, the answer is that the optimum level of greenhouse emissions by humans is zero. That will not happen, but the more we emit the bigger the problems.