The 250 rule is done to research about your niche.
We understand that if you enter a small city, it won't add up to 250. You need to make sure that when you scale to bigger cities, you have at least 250 people searching for your niche.
So to build your master domain site properly, select a major city. Do your research, get the best niche and keywords, and then start building city pages. Start with smaller city pages first and then on to the major city.