Use text-align:justify
on the container, this way it will work no matter how many elements you have in your list (you don’t have to work out % widths for each list item
#nav {
text-align: justify;
min-width: 500px;
}
#nav:after {
content: '';
display: inline-block;
width: 100%;
}
#nav li {
display: inline-block;
}
<ul id="nav">
<li><a href="#">HOME</a></li>
<li><a href="#">ABOUT</a></li>
<li><a href="#">BASIC SERVICES</a></li>
<li><a href="#">OUR STAFF</a></li>
<li><a href="#">CONTACT US</a></li>
</ul>