try this::
x=thisComp.layer(“Null 1”).effect(“Scale”)(“Slider”);
range=200; //range when Z begins
center=thisComp.width*0.5; //center of comp on X axis
distance=x-center;
Zdist=400;
if (x <= center ){
Zpos=linear(center+distance,center-range,center,value[2],value[2]-Zdist);
}else{
Zpos=linear(center+distance,center,center+range,value[2]-Zdist,value[2]);
}
[x,value[1],Zpos]