@GeoffR: Yup, that works for me, too

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At a (very) quick glance, nothing else looks to have suffered from it \o/.
For people also using your adjustable top/bottom margin patch, don't forget to modify the
default margin value to match the one you use in the MediumVertSpacer patch

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I'm using 0 in both cases, and am generally running with the margins slider set to 1, meaning 11px margins on the four sides on the H2O

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EDIT: Ah, just noticed one thing: while the top/bottom margins honor the slider in ePub, they do NOT in the kePub reader... (Unless I'm mistaken, they used to with the old fullScreenReading mode).
EDIT²: I *am* using the 'Custom kepub default margins' patch, if that helps...
EDIT³: While a NTH feature, I can definitely live without it. As I said, I usually stay with 11px margins, so I can just patch 'em to 11px instead of 0.