The only problem with this is that you cannot get a precise decimal from some numbers (e.g. if you have 3 pages, each one would need be 33.3333....% wide). One way this could be fixed would be by having your pages scroll vertically as opposed to horizontally, thus they could all have a comfortable width/height of 100%.
margin: 0; padding: 0;
width: 200%; /* 100 * number of pages %) */
width: 50%; /* 100 / number of pages % */
The site you mentioned is actually not optimized very well for different page sizes. What it does is oversize the pages and center the content (for a certain page width) so that you can resize your browser and you will still be able to at least see the content. However, the layout breaks in an 800x600 layout or larger than 1280x1024.