Nimitrix - thanks! That helps, but only part of the way - I have objects that I want to spawn in all different locations. For example top of the screen +200.
Using your method and knowing my resolution enables me to place things fairly well, but then when screen sizes change it won't work (the screen size is bigger on iPad than iPhone for example. So if I want to spawn something from the top-left corner on both devices, I can't designate co-ordinates in reference to the screen centre. I would need the left and top sides).
One method I used before was to make a sprite for each side of the screen, and on start of layout go:
While 'screen top' is on-screen > set 'screen top' Y to 'self.Y-1'
And do that for each side. It works well enough, however I have concerns about performance by having the extra four objects as it's already scraping in below 30 frames.