Actually, I have yet to unpack a mobi file (purchased from Amazon, or built by calibre or kindlegen) whose paragraph indents weren't specified in whole increments of em's. Keep in mind that there is no css inside a mobi file. Whatever converter you use has to translate the "text-indent" css style attribute to the html 3.2 "width" attribute of the <p> tag.
Now it's possible that someone might manually add entities to adjust the text indent to their liking, but that's not what I'm referring to.
I just built a mobi with calibre starting with an epub that had 1.5em paragraph indents. I told it to convert to mobi, to remove spacing between lines, and set the indent-size to 1.3em. Unpacking the resulting mobi with mobiunpack.py, the 1.3em request as been changed to a 1em indent.
Code:
<p height="0pt" width="1em" align="left">The helicopter was idling. Two men were carrying the stretcher.</p>
All of which corresponds to what I see when reading books on my Kindle... 1em or 2em indents. One's a hair too small , the other... way too big.
