var measure= document.createElement('div');
measure.style.height= '10em';
document.body.appendChild(measure);
var size= measure.offsetHeight/10;
document.body.removeChild(measure);

p {
    font-size:2em;
}

body {
    font-size:0.625em;
}

p {
    font-size:2em;
}

$(window).resize(resize); // update when user resizes the window.

$("body").append("<p id='textSizeReference' style='display:none;padding:0'>T</p>");
var refTextHeight = $("#textSizeReference").height();
setInterval(function() {
    var h = $("#textSizeReference").height();
    if (h != refTextHeight) {
        refTextHeight = h;
        resize(); // update when user changes text size
    }
}, 500);