Exported fbx data errors

Hi Chaps, I was so pleased with myself for working out how to export the camera with fbx from the sequencer in layers, for fusion compositing.
All seemed fine, so I put a box in the scene and exported that as a guide for a greenscreen card. It matches the camera move… but not quite.
Something is off and it drifts. Any ideas what I may be missing?

parenting space?
filmgate size?

Thanks for the ideas.Tried all of them and was about to give up…
When I went back through each node and discovered UE4 had actually exported at 960 x 540!
No idea whats going on there, but resizing clicked it all into place. What relief.
Thanks again!