3D Corner Pin w/ Precomps

Hi everyone,

I've been using Dan's expression for getting the corner pin in 3D. But it's getting tough with the following setup:

1. Main Comp
   1. Camera
   2. Precomp
      1. Camera (pick whipped position and POI with main comp camera and with the same settings)
      2. 3D Null
      3. 3D Null
      4. 3D Null
      5. 3D Null
      6. Solid w/ Corner Pin (using Dan's expression w/ nulls)

It all works perfect when:

1. Both the main comp and precomp are of the same dimension
2. The precomp's position stays centered

Now I can simply fix the first by calculating the offset and adding this to the expression, but the second is the problem. The calculation doesn't work, because of z-space. And by offsetting the toComp (toComp( [0,0,1500] )) it's hard to get right and leaves an apparent offset.

Any ideas for getting it spot on?