Okay I have tried it out and your epub works both in the sigil previewer and kindle previewer standing alone, but when I insert it in my epub it looks right, in the sigil book view mode but when I preview it in kindle previewer, the same thing happens. The text just shoots right up to the top.
Here is a look at the code above and below your code:
Code:
<div style="page-break-before:always;"></div>
<div id="content">
<div class="middle_row">
<p class="middle_cell">Your Text<br />
Centered Here</p>
</div>
</div><!--
FOOD FOR THOUGHT
Did you know that of all the countless creatures that inhabit planet earth, that man is the only one who destroys his food with heat and chemicals before he eats it? And that consequently, man is the only one who needs so many hospitals?
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<div style="page-break-before:always;"></div>