pop up position

Hello (Skandha ?), I used the pop-up plugin. The popup has in the site different views, depending om the operating system you use and the screen format. An example: on my MacBook Pro the pop up is very low on the screen. On a windows desktop the pop up stands in the middle of the page and if I enlarge the distance from the bottom a bit, the pop up hides the menu. Is there a way to have a fixed position for the pop up in all operating systems and all screen formats? Kind regards Hans Hagmeijer