If you get Great Persons through tech (first to monarchy, invention...), you actually get the amount of cultural points currently needed to create a great person.
Say you need 266 cultural points for the next great person, and you are at 166 cultural points when researching monarchy.
When you finish researching monarchy you get 266 cultural points. The first 100 CP completes the great person.
And the remaining 166 points becomes your boost towards the next great person.
Since 400 cultural points is the next limiit to reach, you will have 166 of the 400 CP.