No margin means that no margin is set by calibre during conversion. This means that if there is a pre-existing margin it will be left alone. IIRC, this behavior was added specially for JSWolf.
Doesnt the modify epub plugin have an option to nuke margins?
Also, why dont your friends simply set the margin to whatever they want it to be? That way they dont have to waste time adjusting the margins for every book they want to read on the device.
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