[RbnJrg]

Just now I read your post. I don't know if you've already solved your problem but if you're still looking for a solution, this is mine:

1. In your .css stylesheet, write the following:

.container {
  display: block;
  margin: 1.5em 0;
  text-indent: 0;
  text-align: center;
}

.formula {
    font-family: serif; /* feel free to change this property */
    font-size: 1.2em; /* feel free to change this size */
    font-weight: normal; /* use bold here if you wish */
    font-style: italic; /* use normal here if you wish */
    padding: 0.25em 0.5em;
    background: #FFFFBF; /* change the color if you wish */
}

.number {
  float: right;
  font-family: serif; /* feel free to change this property */
  font-size: 1.2em; /* feel free to change this size */
  font-weight: bold; /* use normal here if you wish */
  padding: 0 16px 0px 10px;
  margin: 0 0 0 -0.2em;
}

p {
  font-family: sans-serif; /* feel free to change this property */
  font-size: 1em;
  margin: 0;
  text-align: justify;
  text-indent: 0;
}

p + p {
  text-indent: 1em;
}

2. In your .html file, write the following:

<p>...Some text here ... </p>

<p class="container"><span class="number">(1)</span><span class="formula">C = π r²</span></p>

<p>...Some text here ... </p>

Of course, your formulas will be different

VERY IMPORTANT: The class "number" MUST BE FIRST than class "formula". That is the key in order that the number and the formula are in a same line.

Below you can see a screenshot of my ADE and the respective epub.

Regards
Rubén