Price discrimination- how much is optimal?
Varian has a paper on Price Discrimination and Social Welfare in which he gives some necessary and sufficient conditions for (third degree) price discrimination to increase welfare.

A necessary condition is that the total level of output (i.e. the total number of consumers served) increases as a result of the discrimination. 

A sufficient condition is that the profitability of the new output (i.e. after discrimination) exceeds the profitability of the old output (before discrimination) evaluated at the new prices.