Default user space vs Rotated space

Hi - I am hurting my brain trying to find a consistent relationship between default user space (dus) and rotated user space (rus).

I know of the transform to use for dus -> rus and vice versa. But I am missing something which seems related to the way the file was printed to pdf.

My task:
Position a annotation box on a pdf document in approximately the correct position where the user should sign. The user can move/size the annotation box into the exact position and press a button which then transforms the annot coords to rotated user space, creates a dig sig field, and signs it for the user (user having previously provided his digital ID and p/w). This works fine, but determining the default user space coords for the annot box is problematic.

This process seems inconsistent between different page sizes and the way the file was printed to pdf.
i.e.
For an A2 landscape file (seems printed as A2 landscape), the dus coords are the same as the rus coords. All works fine.
For an A3 landscape file (seems printed as A3 portrait) the dus look correct (rotating to portrait shows the correct dus coords) and transforms to a new set of rus, but the annot box is wrongly placed.

I can fudge the coords but I would like a consistent programmatic process which works irrespective of the page size and print process.

Ideas?