I'm seeing this behavior too, odd. I don't see anything wrong with your code right off hand (hey, just have to check before filing a bug ;)) and looking at the IL for the RichText.set_ZoomFactor (and a subsequent call to a private method) doesn't show any calculation bugs either. I'm guessing this is actually a problem with the Rich Edit common control somewhere in the EM_SETZOOM message handler. So, googling for "Rich Edit" EM_SETZOOM[^] shows you're not the only one! Feel free to report, though. Maybe someone hasn't yet. Never know.
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