Problems resizing a slab that is being composited with video in fullscreen

Hi - I have a resizing / fullscreen issue.

I have a patch in which I pull in an app I wrote in openFrameworks via syphon. It gets put onto a slab, which is then composited with a 16:9 video. This matrix is then composited with another video and chroma-keyed, so that you can remove parts of the foreground video to reveal the background video. The whole thing is put on to a videoplane, which is set to @transform_reset 1 @scale 1.77777 (16:9). This allows the videos to maintain their proper 16:9 format (960x540) in full screen mode.

The problem I'm having is that the slab with the syphon content gets stretched wide, and I'm not sure where or how to resize it in a way that the videos don't also get stretched, as they are exactly how I want them. I've attached the patch - the part to pay attention to is the lefthand side (the bottom left box) - which is where all the compositing and resizing happens.

If anyone can point me in the right direction, that'd be great. I've tried a lot of combinations to resize things with no success so far. I should point out that I can also set the output size of the syphon content (by default it is 640x480) - but so far changing its size hasn't seemed to matter when the content goes fullscreen.