Most I've ever got is 210 (I think). Done it by playing as the Americans, building a city next to 3 mountains, built an iron mine and a factory (the x3 production really helps), changed to communism, built settler units in other cities, bought them to increase city population rapidly, then I got the acheivement and went back to Democracy. Might of been able to get to the 250 if I reached the maximum 31 population and researched future technologies.