The thing with Calibre though, while it does convert from epub to Mobi pretty much, it also has a bug (or design feature, as people have been complaining about it for a long time and hasn't been altered) that pushes the margin over far too much.
See
https://www.mobileread.com/forums/sho...&postcount=330
For what I mean.
So you have to open up the epub and fiddle with the css. If you have a lot of epubs, that could be a lot of work. It depends on the epub though.