$(window).load(function () {
    var mf = $("#MainContent_mainFrame");
    var winHeight = $(window).height() - 70;
    mf.height(winHeight);

    $(window).resize(function () {
        var mf = $("#MainContent_mainFrame");
        var winHeight = $(window).height() - 70;
        mf.height(winHeight);
    });
});