Try this instead. Better compatiblity and more robust.
<div style="position: absolute; top:50%; width:100%; height:1em; margin-top: -.5em; color: white; font-size: 10px; text-align: center;">My Text Here</div>
The above should align the text exactly in the center of the page. The trick is 'text-align: center;'. All child text of a tag will be automatically centered by the browser.
Also, just a note: its usually considered bad practice to use deprecated tags such as the font tag since its results can be achieve using CSS. I refrained from using it and used its general equivalent in CSS.