Does the 100ml per film mean also 100ml per sheet? What if I overdevelop for double time, do I have to take that into account in replenishing volume?
The standard replenishment is 70ml per "8x10 or equivalent" -- that's a 135-36, 120, four 4x5, two 5x7, or a single 8x10, near enough. And no, longer development doesn't change that amount (though in theory, more exposed or less exposed film might, Kodak's documentation doesn't call for any changes for that, either).
As I understand it, the reason this works the way it does (no adjustments to replenishment based on film type, exposure level, or length of development) is because self-replenishment with Xtol "over-replenishes" compared to other replenishment systems like, for instance, Flexicolor. I use the LORR (LOw Replenishment Rate) color developer and replenishment is typically less than half what Xtol would call out for the same amount of film; in addition, more is called for with ISO 400 or faster film than with slower films. But Xtol stays much closer to the state of unseasoned stock solution; fully seasoned Xtol in steady stated of replenishment is about the same activity as stock solution reused without replenishment for four rolls in a liter, and will stay that way through multiple 5L packs of replenisher (if everything else cooperates).