scale expression

Hi

i've got a 3d comp splitted into pieces with i.e 10 different masks.I then used a script to separate all mask into its own layer.

So,i basically have now 10 layers that form this comp now.

when i now move i.e one layer into z space i'd like that the scale property creates the illusion to keep the layer in place.

the layers moves in z space and the scale expression reacts accordingly to always fill the gap.makes sense ;-).

In the front view it looks as if the comp has only one layer.

now i'm looking for an expression that calculates this,any ideas?