I could manage to (partially) solve this problem for my books by overriding the paragraph CSS before conversion.
1., Convert- set output to MOBI
2., Look&Feel>Extra CSS textfield insert
Code:
p {
text-indent: 1em;
display: block;
text-align: justify;
margin-left:0;
margin-right:0;
margin-top: 0.5em;
margin-bottom: 0
}
or whatever you prefer. Overriding the <body> tag didn't work for me.
In case of a well formatted input book it should produce an acceptable output.
Sometimes I change the <h1-5> tags, as well, setting the margins and padding to 0. Otherwise manual editing
or you should know the CSS class names within the ePub book (IOT override them). It's possible using debug mode.