The downside of Calibre is that some epubs won't convert correctly as is, the left margin gets fouled up.
See:
https://www.mobileread.com/forums/sho...&postcount=330
For an example. Apparently happens if the person who made the epub set a left margin, so you have to edit the CSS file in the epub to set it to zero (or just delete the CSS, which is what I do)