Rav Moshe explains the 600,000 opinion as a daily fixed number for a stand alone road, like a highway. And for a city it would be the amount of people that are around and about in the streets. Meaning, that Rav Moshe’s opinion is that the number is only about reshus harrabim itself and nothing else. So for a city, those that are inside wouldn’t contribute to 600,000.
Rav Moshe says that the population would be estimated to produce the amount on the streets, and it is likely that different cities vary on this calculation. Also, the area would be 12 mil square [slightly over 7 miles] (less than 9 miles) like it was in the dessert encampment. So then if one would measure in such an area enough people to have 600,000 in the streets then it would be a reshus harrabim for sure. [And Rav Moshe would have told the Rabbonim that there is no way to make an eruv without dalsos and so on.] But if there is a street anywhere in this vicinity that carries 600,000 by itself, than that street alone would be a reshus harrabim for sure.
Rav Moshe has no intention of counting the city as was evident in his first teshuva. But if one would count and have 600,000 in the streets than Rav Moshe would have forbade the eruv. And it would be null according to his opinion. Some people use this paragraph as way to calculate, but that is not the point. Such would only achieve that it is not a reshus harrabim beyond any doubt. It wouldn’t rule out a safek doraissa and it ignores all the nuance of the first teshuva. Additionally, there is no statistical rule here. Rav Moshe lists five different activities to count. In two different places. And three estimates. Since it has no practical application, there is no reason to be clear about it. The point is to get the idea.