@Dave...

Parallel Projection.
I see that it makes a perspective difference. Thanks for that.
Profile updated.

I have done quite a range of tests and have a consistent import scale of 147.05% coming into Illustrator. Why? Am I missing something else?

I found that putting a bounding box around my test shapes helped test the sizing accuracy – and give me some confidence. Although I am still not locked into a process.

Has anyone worked with export for pattern printing?

I have been using S'up for many years but would hardly consider myself an expert. I have been doing mostly small architectural and furniture modelling. And now I am planning several lightweight hiking tents. There is curvature involved and I have been using Fredo6 Curviloft with good results.

I have been using Alex Schreyer: Unwrap and Flatten Faces which also works well, dropping a panel to ground.
Currently using s'up version 20.
Also have s'up Make and my old trusty ver 6.4 from 2007

The problem is translating a S'up panel into a full-size pattern.
I draw a rectangle: 800x333mm at 0, 0, 0
Camera: top view
Export as dxf/dwg (full size) and get a rectangle 544x226 – which is correct proportion but ain't 800x333. And of course it gets more complicated with irregular shapes. Perhaps there is a consistent scale that I can set when importing (into Illustrator)?

The other option is to dimension the life out of the flattened panel and manually draw it (yawn).

I am just getting started with this so would be really interested to hear any suggestions.

Thank you.