Problem centering a MC since the use of Z axe

Hi everyone!

I’m playing around with the Z axe for a project of thumbnails gallery and here’s my problem:

Let’s say I’m centering a square (register point centered) in the middle of my screen, and I want it to keep being in the center of my screen when the stage gets resized. No prob.

But since I added a depth axe with something like mySquare.z=600, when i’m resizing, my movieclip doenst realign correcly and seems to stay in the 1/3 of the X / Y axes.
I dont understand why this is happening as I’m not chaning anything else.

Can you help me ?

Thanks a lot!!