Once you have the KF8 ready to go other then adding the negative margins, just open the eBook in the Calibre editor. Then in the main CSS file, add the following to the top of the CSS file and save the eBook.
Code:
html {
margin-right: -45px;
margin-left: -45px;
}
Then you are done. It does work for me. It's what I do and it's what works. I do make sure there there are no left/right margins for normal paragraphs as they don't need any.