The formula of the "turning point" in a Kuznets curve (where the dependent variable reaches its maximum value) is exp(-ß1/2*ß2). ß1 is the coefficient of the linear GDP and ß2 is the coefficient of the squared GDP.
For a single country, this turning point is of course suitable. But does it still apply to a cross-country panel regression where different countries have very different GDP levels (but we only have one turning point)?
Many thanks!