menubar length-css

I have played around with the css to get the menu bar to look right on my ipad. Problem is now that the menu bar has some white space on the right end when viewed on other monitors. I seem to be able to vary the size of this white space by dabbling with the css but am not able to get rid of it entirely, without affecting its appearance on the ipad screen. the site is <arnoldgenealogy.com> the custom css: } #header #mainmenu ul li:last-child { border-right: 10px; } #header #mainmenu ul li:last-child a { padding: 0 26px; }