@Barty: I vaguely remember posting a FontForge screenshot of what the Kindle renderer actually cares about when tweaking these things. With a bit of luck, I linked to it in the FAQ. If not, try doing an attachment search on my nick, the screenshot would have been a PNG.
But, yeah, what @shamanNS said holds true, I don't remember how the mobi6 renderer behaves on that front, so I'm talking about KF8 here

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And potentially forget about KFX, too, since that's a whole other can of worms...

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