Forgive me if I don't use the correct terms, but I'll try to explain my problem. I have layer "A" positioned in the middle of the composition. Layer "B" is also located in the center of the composition but offset in z-space. When I move layer A in the x-axis, I want layer B to follow, but with an extra movement. For example, if I animate layer A 100px to the right, I want layer B to move 150px to the right from the center. And if I move layer A 100px to the left of the center I want layer B to move 150px to the left.
So if layer A was animated from -100 px from composition center to 100px from center, layer B would move a total of 300 px and layer A 200 px.
if you can use a null (or other layer) as a parent to both layers A and B, and you make layer A's average position relative to the parent (so it's x position ranges from say -100 to 100, or -200 to 200), then this simple expression would work:
target = thisComp.layer("A");
x = 1.5 * target.position;
essentially, you're using the parent to zero the position of both layers to make the math easy.