[KevinH]

I see nothing obvious but controlling the letter spacing (sometimes negative!) and kerning and mixing css stylesheets with styles in the head tag and styles attributes directly on the tag is asking for trouble. Not to mention wrapping each paragraph in a div that may have its own styles.

There is one tool in Sigil that might give us a hint. In Sigil's Preview show the problem area. Then invoke Preview's Inspector tool and highlight the problem span and it will show the css properties of that block including where each property came from.

That should give you a hint to see if really a justification issue or if the right margin setting or padding is incorrect, or is the containing div's width is the issue and where that property setting is inherited from.