Today worked on getting a real reference for the correct proportions/size matching the artwork.
My approach was first to use my human reference (1.80 m) to guesstimate the position (if anyone has a pointer to a more exact/accurate method, let me know 🙂 ) and then match the proportions starting with a cube, sizing rotating and moving until it matched the reference in camera view. Result is this for now:

Next I checked with the help of @theluthier ‘s house blockout whether the position made sense in 3D, which ended not llooking too far off. I guess some final size adjustment will probably be needed once the final breakdown file is available, but at least the proportions should be close enough.