In a div2 only round, when the person with rating 2099 is the first, he jumps 200 points or more. However, if he is 2100, he does not get anything. Therefore, I suggest a gradual division boundary. First, we calculate the standing including every person that his rating could change and calculate the rating.

In div2, if his rating is lower than 2000, it is fully rated for him. If his rating is between 2000 and 2200, he would get ratingchange*(2200-rating)/200 ratings. If his rating is greater than 2200, he is unrated and doesn't include in the standings. Same thing for div3 and div4, with different bound. However, I think it should not be done in div1+2 separated round since 1900 could just join div1.