How to position center customized logon screen in web Forms 11gR2?

Hi,

I would like enter align application login webpage(syslogin.fmx)in I.E browser applet window

I try with below code, code is working, but form is not exactly in center of the window, please assist me.

DECLARE
screen_height NUMBER;
screen_width NUMBER;
window_height NUMBER;
window_width NUMBER;
applet_size varchar2(30);
BEGIN

applet_size := webutil_browser.getappletsize();

screen_width := substr(applet_size,1,instr(applet_size,',')-1);
screen_height := substr(applet_size,instr(applet_size,',')+1);

window_height := GET_WINDOW_PROPERTY('LOGIN', HEIGHT);
window_width := GET_WINDOW_PROPERTY('LOGIN', WIDTH);

SET_WINDOW_PROPERTY('LOGIN', X_POS, (screen_width - window_width)/2);