function getInsideWindowWidth() {//ok
	if (isNav4 || isNav6) {
    return window.innerWidth
	} 
  else 
  {
    return document.body.clientWidth
	}
}

function getInsideWindowHeight() {//ok
	if (window.innerHeight) {
		return window.innerHeight
	}
 else 
 {
    return document.body.clientHeight
 }
}

function checkHeight() {
  windowHeight = getInsideWindowHeight();
  obj = document.getElementById('inhoud_div_id');
  objMenu = document.getElementById('menu_div_id');
  objHeader = document.getElementById('header_frame_div_id');
  objFooter = document.getElementById('bottom_bar_div_id');
  ratio = 1/12;
  newHeight = windowHeight - objMenu.offsetHeight - objHeader.offsetHeight - objFooter.offsetHeight;
  newHeight = newHeight * ratio;
  newHeight = newHeight - 4;
  obj.style.height = '' + newHeight + 'em';
}