Here's an easy way to make your navigation items stretch to the full width of the container (nav) evenly.
It doesn't matter what the parent container width is, you want the items to use the full width.

HTML markup
<nav>
<ul>
<li><a href="#">Item 1</a></li>
<li><a href="#">Item 2</a></li>
<li><a href="#">Item 3</a></li>
</ul>
</nav>
Code language: HTML, XML (xml)
CSS Tables
I know what you're going to say but I've found that this method has great browser compatibility. It just works.
nav {
display: table;
border-collapse: collapse;
}
nav ul {
display: table-row;
}
nav ul li {
display: table-cell;
}
Code language: CSS (css)