If you want the dialog to hover over your main front panel in the same position you will probably have to pass the main FP coordinates. Another possibility is to place a "sub-panel" control where you want the sub-vi to display, making the frame and scroll bars of the control invisible. Then when you want the dialog vi to be visible have it opened by the sub-panel controls methods on the diagram. You could also have the main always centered on the screen, and then set the sub-vi to be centered as well. Doesn't actually track the position of the main though.
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