You could just make the outer box absolute, if your textbos has to be positioned absolute.
EDIT: Without being able to edit the HTML structure, you need specific heights or some JavaScript. More Information about position
.box {
text-align: center;
border: 1px solid red;
width: 500px;
margin: 0 auto;
position: absolute;
}
.content {
width: 50%;
left: 0;
right: 0;
margin: 0 auto;
}
<div class="box">
<div class="content">
Lorem ipsum dolor sit amet, consectetur adipisicing elit. Assumenda adipisci vel, dolore aspernatur iste iure blanditiis quam esse repudiandae aperiam debitis doloribus necessitatibus placeat tempora voluptate totam exercitationem neque quae.
</div>
</div>
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