Just
look at the minimums and go from there. Depending on climate it's way less than a ton of Carbon per capita. The richer countries tend to use a lot ten to a hundred times what the poor countries use, and even among the wealthier countries there are large gaps. Emissions in France are a third of what they are in the U.S. but they aren't exactly scraping by at sustenance levels. Given current tech I imagine ~1 ton per capita is a reasonable worldwide goal.