You have a couple of things going at once, calibre11 and text-align:center. The latter is enough. calibre11 has a text indent of 1.5 ems. That is throwing it off.
What you might try is to save a copy, use search and replace to remove all of the calibre styling and see what is left and how you like it.
If you have html to start with, take it directly into Sigil by adding additional files or opening. That way, no extra stuff from calibre.