move a layer from a slider control???

Hi!
I am trying to create some kind of balance control, my idea was to create a knob that would move in a half circle... I have not yet found that it is possible so I was trying to "fake" the behaviour...
I provide an image link as it is hard to explain with my bad English

the yellow part (prototyping)
the square is a background
the half cirkle is a balancelr
the black line below half cirkle is a sliderbit_normal
The yellow part represent the lower half of the volume pot at right of display... [open image to see full, forum cuts it]
The idea was to make a hole where the sliderbit_normal would be visible, faking a balance marker (could work if the hole is thin enough). Problem: the sliderbit_normal center on balancelr so it sticks out on top and bottom. I thought of fixing it by using a link(layer_id) thing... I created the black line and made a link from balancelr as I thougt, if I move the slider, I also move the layer_id... It did not...
So? my question is: how do I link and move a second layer from balancelr? is it possible?
I am trying to create some kind of balance control, my idea was to create a knob that would move in a half circle... I have not yet found that it is possible so I was trying to "fake" the behaviour...
I provide an image link as it is hard to explain with my bad English

the yellow part (prototyping)
the square is a background
the half cirkle is a balancelr
the black line below half cirkle is a sliderbit_normal
The yellow part represent the lower half of the volume pot at right of display... [open image to see full, forum cuts it]
The idea was to make a hole where the sliderbit_normal would be visible, faking a balance marker (could work if the hole is thin enough). Problem: the sliderbit_normal center on balancelr so it sticks out on top and bottom. I thought of fixing it by using a link(layer_id) thing... I created the black line and made a link from balancelr as I thougt, if I move the slider, I also move the layer_id... It did not...
So? my question is: how do I link and move a second layer from balancelr? is it possible?