$(document).ready ( function() {
  var item;
  var menu = 1;
  var j;
  $("#menu-principal li").each(function (i) {
    i+1;
    item = $(this).width();
    //alert("item " + " " + i + ": " + item);
    menu = menu + item;
    j = i;
  });
  
  //alert ("menu: " + menu);
  
  
  j = (j+1);
  a = j * 7;
  b = (j-1) * 2;
  
  sobrant = (760-menu-a-b);
  
  
  marges = sobrant/j/2;
  
  $("#menu-principal li a").css("padding-right",marges);
  $("#menu-principal li a").css("padding-left",marges);
  
});