var hashScrollFix = function() {
    var hash = location.hash
      , menu = $('.main-navi.fixed') // your fixed menu selector
      , itm;

    // I added menu.position().top >= 0 because I fade my menu to above viewport
    // with a negative top position
    if ( menu.is(':visible') && menu.position().top >= 0 ) {
        itm = $(hash);

        if ( !itm || !itm.offset() || !itm.offset().top )
            return;

        var jump = function() {
            // adjust scroll
            return $('body').scrollTop(itm.offset().top - menu.outerHeight());
        }

        // on load
        jump();

        // chances are we may need to jump again
        setTimeout(jump, 100);
    }
}

$(window).bind('hashchange', function() {
    return hashScrollFix();
});

var makeMyMenuFixed = function() {
    // .... do things that render a fixed menu

    // then adjust scroll
    // if you are using animation, then you might want to wait until it is done
    // e.g .animate(.., .., hashScrollFix)
    hashScrollFix();
}