My CSS skills are basic

I am trying to mimic the printed version.
I have an Image that is used on every chapter
On that (in the lower 1/3) I want the chapter digit (text). see attachment
The problem is if I re-flow (zoom) on my reader text, the digit moves out of place (
ugly).
How do I stop the re-size of just this text (64 Images is not the answer type I want

)
Existing coding :
Code:
/* Overlay Chapter digit */
/* the code withe the problem */
<p class="calibre5"></p>
<div class="overall">
<p class="big1 calibre8 middle"><br />
<br />
1</p>
</div>
<p class="calibre8 sgc-2"><br class="calibre2" /></p>
/* ========== the CSS pieces ============*/
div.overall {
display: block;
background-image: url("../Images/image02.gif");
background-repeat: no-repeat;
background-position: center;
height: 424px;
width: 180;
position: relative;
top: 10
}
div.middle{
position: relative;
margin-top: 5
}
.big1 {
display: block;
font-size: 5em;
font-weight: normal;
margin-bottom: 0.5em;
margin-left: 0;
margin-right: 0;
margin-top: .5em;
text-align: center;
text-indent: 0
}
.calibre8 {
font-style: italic
}