Dynamically parent two layers together with expressions

Question

In AECS6, I have an Image Layer that is using the layer directly above as a Alpha Matte. I want both layers to scale at the same time, unless a certain condition is setup where I only want the matte to scale - leaving the image at original size. Normally I'd just make a new pair of layers and only scale that, but ALL my layers are dynamically linked to eachother for another more complex effect.
I wrote a simple expression to get the current scale of the matte layer (index-1), and then calculate what this particular layer's scale should be. But the scales don't match up just right because we are dealing with percentages... not absolute values. However, if I simply parent the two layers together, it of course works.
So, I either need to find a way to turn the parenting on/off with code, like if THIS, then ParentTo(index-1), or I need to write my scaling script correctly.
Fwiw- here's that script:
if (index != 15)) {
  // get the original and current scale value of the layer just above, and calculate an offset
          sOrig = thisComp.layer(index-1).transform.scale.valueAtTime(0)[0];
          sNow = thisComp.layer(index-1).transform.scale[0];
          sOffset = value[0] + (sNow - sOrig);
          [sOffset,sOffset]
} else {
// this is layer index 15, don't do anything with it
          value;
Appreciate the help!

Answer 1

Not sure what you are getting at, but the actual value ranges do not really matter in an expression. It's more likely you are eitehr having an anchor point issue or your keyframes are simply not linear. in the latter case of course a simple subtraction would not give the correct result because values would be exponential/logarithmic/whatever. You'd have to use much more complex code then to accumulate all values over time such as the great Dan Ebberts explains on his website:
http://www.motionscript.com/articles/speed-control.html
Anyway, I suppose you could always use effects rather than native transforms to scale your matte while still leaving it parented. Y' know, there is a Transform effect... That would be much simpler and also avoid the expression evaluation bogging down your system...
Mylenium