Hmm not sure, hard to visualize. Sorry I was pretty tired when I wrote that, maybe it doesn't work.
The idea is to take the difference between the area's total width or height, and where the first object spawned.
Imagine 0 to 10 on a line, where 10 is your total width.
Opposites would be 1 and 9, 4 and 6 ect. So if my object spawned at 1, 10-1=9. 10-4=6.
If you do this for both the X and Y you should get the other side of a square. I think.