Hi Martin,

re:
> It seems the correct bounding box dimensions are available in the Edit Size box, but MoI is not using them
> in this instance. The linked video shows the problem quite well.

Thanks for reporting this bug, I can repeat it over here and have put it on my list to investigate.

For now to complete your drawing you can use the BoundingBox command followed by Transform > Mirror to get the precise mirrored result.

- Michael