600,000 IMO would be possible. However, it would need to be an effective conquest. Raze everything after about 45% land. Make sure to capture every wonder giving free culture. In the far reaches of the empire city spam a ton of 1 apart cities. The main goal is all the free culture. We would need to reduce the world to a handful of cities. IIRC we were +8 culture in every new city. That is 4,000 a turn with 500 cities. That is just 150 turns. That is just from the free culture. The first thing we need to decide is if we try for the new single city culture, or civ wide cultural goals. Please post when you have the new version installed.