Have you tried setting the li width to, say, 16% with a margin of 0.5%?
nav li {
line-height: 87px;
float: left;
text-align: center;
width: 16%;
margin-right: 0.5%;
}
edit: I would set the UL to 100% width:
nav ul { width: 100%; margin: 0 auto; }