Originally Posted by kovidgoyal
No, it's a limitation of the MOBI format. You can only get left margins in increments of 2em on the Kindle with MOBI.
It may be a limitation, but doesn't change the fact that the conversion results in books that are an eyesore to read on a Kindle.
Love your software, but this is rather annoying. Programmers have to work around bad format limitations, and in my humble opinion the conversion from epub to mobi in Calibre should account for this limitation and remove the left margin values.