This image (due to some aero/messenger error) shows more clearly what pieces are missing....
EDIT: I've fixed the top left corner now, had that at 50% width and height still, but the others remain unfixed.
I guess the main problem is that we can't "center" the mask pieces and I'm trying to tell it to align to both the left with a 15pxl padding and the right with a 15 pxl padding
and it only want to align/pad one side.
ALRIGHT!
Managed to do what I was after using Subtract instead of add.
UMargins, would still be REALLY nice though