The exact details don't matter, these are just guidelines.
I looked at a few discusions from the CreateSpace community. The most helpful one is this one.
You can see the different recommended inner margin values:
- up to 150 pages: 0.375"
- 151 - 400 pages: 0.75"
- 401 - 600 pages: 0.875"
- more than 601 pages: 1"
But it's important to understand what the reasoning behind these values is (emphasis mine):
You need to keep in mind that the thicker the book the larger the inside margin needs to be. That's because the inner part of the page will be more difficult to see on a thick book unless the margin is sufficient. If there's not enough margin, the reader will tend to open the book wider taking the chance of breaking the book's back, if you will. A 150 page book will not need an inner margin as wide as a 400 page book. Likewise, a 402 page book shouldn't really need inner margins larger than a 400 page book. The page ranges versus the inner margins are merely guidelines.
If CreateSpace gave you a formula for calculating how much margin you need per extra page it would be far too complicated as it's just not necessary to obsess over a couple of pages more or less. They give you rough guidelines that will work for these values.
That means that if you expect to end up close to 400 pages it's better to be safe and choose the bigger margin that belongs to the "401 - 60 pages" range. It wouldn't be a problem to simply choose something between those two values if you are afraid of wasting space. ~410 would likely still work just fine with the smaller margin of 0.75", but it would be better to at least go with something like 0.825". When you are at ~500 (+/- a couple) pages you should switch to 0.85" and when you are getting close to ~600 pages you should use 0.875".
If you choose your margin to be too little your readers might have problems actually reading your book. If you choose your margin too big the worst that might happen is that you might need a few pages more for your book and it becomes a little bit more expensive.
If you are unsure: choose the bigger margin.