I’ve created a “Leader” object which moves in Y position, and attached a slider effect renamed “post” to it.
Then, in the layer above it I wrote this expression under the position property:
(see below)
The problem is, it copies the X & Y properties exactly. What I want to do is mimic the change in value of Y of the Control layer while retaining its own separate Y value. Does that make sense?
Thank you so much for your help!

post=thisComp.layer("CONTROL").effect("post")("Slider")
thisComp.layer(index+1).transform.position.valueAtTime (time-thisComp.layer("CONTROL").effect("post")("Slider"))