1. Opening to Kilika Temple (Ifrit acquired)
2. Leaving Kilika to Mi'ihen Highroad (end)
3. Operation Mi'ihen to Moonflow
4. Guadosalam to Macalania Temple (Seymour)
5. Macalania to Bevelle escape
6. Calm Lands to Gagazet (Seymour)
7. Zanarkand
8. Sidequests part 1
9. Sidequests part 2
10. End
The vast number of things that falls under the category of sidequests is going to take a lot of time, whether it's capturing 10 of every monster to unlock Nemesis, beating the Dark Aeons, or what have you. I think making that two is a good idea on its own. That being said, the last part, which obviously has to be the end of the game, is going to be by far the shortest, because from the time you start fighting Sin to the actual final boss is the shortest section of game...in the game.
Alternately, just be aware that your chapters 3 and 4 are very long in terms of game time
, though the rest looks pretty solid (esp the last few).