With that, you'll need to get another pokemon native to the Johto region's caught data. Caught data is expressed by a pokeball icon on the pokemon in your pokedex. Seen data does not count as caught data. 152 to 251 are the pokedex numbers of the pokemon native to Johto. Hope this clarifies things for you. Let me know if you need anything else.
Thank you so much! I've continued the story in Hoenn and will catch the last one as soon as possible! <3