with this one:
<div style="padding-top: 30px; padding-bottom: 0px; height: 559px; width: 99%; margin: 0px -13% 0px auto;">
This seems to work and the form looks nicely centered at all viewport widths. [/b]
You can play around with the parameters and alter the numerical values to your liking - if you so wish. The height in the original is continuously modified by the javascript, as the viewport is resized. I think you could remove the height declaration entirely, since the size of the form will determine the height of the div anyway.
Make sure to take all that style information there and put it into your own class in your CSS file rather than inline like that unless you already have a lot of inline styles in your page already. Yes inline styling works, but it's not the handiest way for editing in the long haul which is why CSS was born to begin with. For this, it is probably not detrimental that it's in the CSS since it's only used in one space in your entire site on one page, but overall your styles should go in a CSS file rather than in the HTML itself.