I have been wondering the same this. I was going to start a topic until I saw yours. I run into a slightly different problem though. I'll use my last show for example. I was using a 9.375mm LED product to makeup columns. The columns were 64px by 640px. Where the problem was was the columns near the end of the images. I didn't have enough shift of the x offset to get the content correctly mapped on there. So in the end I had to trick the Catalyst and make those columns 256px wide, and then the content would make it.

I would love to see an exact mapping so my 64px mix window would be 64px of 1920px, or however big your content is.