Let me just add that converting epub to mobi in Calibre is far from perfect.
If the epub specifies a left margin, it's probably going to be screwed up when Calibre converts it.
See
https://www.mobileread.com/forums/sho...&postcount=330
For example (it actually looks even worse on a Kindle). You can edit the CSS (or just delete it is what I do) in the epub to fix the problem, but if you've got a lot of books with it, it becomes a hassle.