BuddyPress.org

Ok I see you have position relative set for both on the header, this allows for some handy position absolute inside.

Set the margin on the #nav and #nav li to 0 and leave all the other properties, then set the nav to position: absolute, and top: -5px, right 20px(or your pref from the right)

#header ul#nav {

margin: 0;

position: absolute;

top: -5px;

right: 20px;

}

#header ul#nav li {

marign:0 0 0 18px;

}

This should make it more consistent instead of fighting with negative margins.