Well, the position numbers are going to be based on the center position of your graphic…semi assuming it is perfectly centered on the screen in the original document. If you graphics are all the same height, and all centered on screen, then typing in the same position number should work. If they are not all the same height, or not all centred in the document, then you either have to eyeball it or rebuild your psds, to make the bottom of each graphic be at the center line on the screen, or, put the bottom of the graphic at the bottom of the screen.
In my experience, its just as easy to make a line in the text tool, put that on screen, in the bottom position you want to line up, and then put the graphics on upper video layers, justifying them by eye, to that line. Once the graphics are lined up, delete the line.
Glenn