function moveLettre() {
  if ( self.innerWidth )
    largeur = self.innerWidth;
  else {
    var ie = ( document.compatMode && document.compatMode != 'BackCompat' ) ? document.documentElement : document.body;
    largeur = ie.clientWidth;
  }
  x = Math.round( ( largeur - 945 ) / 2 );
  if ( x < 20 ) x = 20;
  document.getElementById( "lettre" ).style.left = x+"px";
}

moveLettre();