In Sigil and in Calibre the width of the cover.jpg is defined in the <img> tag, not in the stylesheet.
In Sigil it is simply width="100%", in Calibre it is like style="width: 100%" (actually it is height).
You could try the same with your ornament.
in HTML
<p class="img"><img alt="verse break" width="38%" src="../Images/ornament-black.jpg" /></p>
in CSS
p.img {
text-indent: 0;
text-align: center;
margin-top: 0.38em;
margin-bottom: 0.38em;
}
As to: "the top margin invariably ended up bigger than the bottom margin"
I think, it has nothing to do with margins, but with the size of the ornament. Probably the height is less than that of the font-size. Because the orientation of the graphic is at the basis of a line, you will get more space above than below. See it in the sample with some text to make it clearer.
You could use the same ornament but in a higher image with some more space at the bottom and less at the top, in other words: lifting it a little bit within the picture. That would compensate the too big margin to the preceeding paragraph.
George