not able to set [x,y] position in dialog

Hello to everybody!
I am having trouble setting up the dialog box to a custom height.
something like ...
$(function() {
    $("#dialog_store").dialog({
        bgiframe: false,
resizable: false,
position: ['center','80'],
    });
});
it will only work with values like top , center ....
So how can i position the dialog 80 pixels from the top?
Please help
Iomas