Expression Issue - Photo Slideshow

Question

Dear forum,
I need some advice on expression settings in After Effects.
The project I am working on is a slideshow with 300 photographs. They move from left to right (panning horizontal while maintaining their vertical position) within 150 seconds.
All of these pictures are in one composition as single layers, in their right order (first layer is picture 1, lowest layer is picture 300). I want to achieve the movement using a null-object. Now between each of these photos there ought to be a space/border of 600 pixels.
I already repositioned half of all the layers and linked them to the null-object. In order to having just the null object as my panning move for the last step.
Now I figured: Why not realise this with an expression. My idea is to give all the layers' position values an expression, that tells them to multiply the value by it's index (layer) number and by 600 (for the space between).
Could you guys give me some advice on how to write the expression code for this.
t.i.a.
Daniel

Answer 1

X=thisComp.layer(index-1).transform.position+width+600;
Y=value[1];
[X,Y]
Mylenium