Sounds reasonable to me. Hell, I've no idea what I'm doing so what do I know?
But my advice, for what it's worth, would be to work out what you want to happen on the way and then put those into chapters. That would give you a better idea of length and balance than the other way around.
My other top tips are:
1) Only make the game you want - otherwise you'll stop enjoying it. There are points where it is not much fun at all, and making something you have no interest in would make that so much harder.
2) Prep your assets in advance for all of the game if you can. It's a bitch having to search for a location midpoint.
3) Have a chapter on the stocks. By that I mean don't release One until Two is ready. This was my original plan and I so wish I had kept to it but a sudden rash of sci-fi games started popping up at the end of last year and I panicked...