I think I've managed to figure it out. Instead of using px or % margins, I've just set the table widths and margins to auto, which spaces it in the center of the window no matter what size :)
I've been trying to get my homepage (www.andrew-hayward.com) to work with a table, but for some reason the picture ends up looking like it's a row above the text. I'm using this code:
<td style="text-align: left;" width="50%">Welcome to my Engineering portfolio website.Details of my achievements and personal interests can be found in the "About Me" section.Examples of both my design and analytical work can be found in the "Projects" section.If you have any questions or comments please contact me.</td>
<td><img class="alignright wp-image-389" title="Compressor Design" src="http://andrewjhayward.files.wordpress.com/2012/01/1-copy21.jpg?w=300" alt="Compressor Design" width="300" height="242" /></td>
Any ideas where I'm going wrong?