The MM, actually. This is why the Tarrasque has a BAB of 48 instead of 34, for example. The exact location uses a Titan as refrence, and then says that iteritive attacks cap out at 4.
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The section from the DMG (Not MM I think) makes a specific note about BAB but says nothing about saves. As you use Epic progression unless otherwise noted for creatures with a character level over 20 and saves from RHD arent otherwise noted the tarrasque should have 20 HD of poor will saves and 28 HD of good.
My argument is that the term Character level is used interchangeably by the authors (probably unintentionally) to mean total of your Class Levels and Total HD (from Class and Race).