Just look at the population to be massacred numbers. You massacre like 70 or 80% so figure it out with that. If the population to be exterminated is 20000, that means there's about 25000-30000 people which means it's gonna be a huge bitch to keep the population in line. Generally I enslave anything above 8000 or so and exterminate any city at 15000-20000 or higher. It depends on the city's position and if I need it to start churning out a lot of troops quickly or not.