Getting Y-offset on systems to be exact on left and right "mirrored pages"

[Owain Evans]

I'm arranging for a beginner piano duet where left and right pages mirror each 
other:
same bars per system, same system alignment-distances, same Y-offsets on each 
system.

I would like the same Y-offset value on left and right pages to be at the same 
level. 
Here it's the notes above the staff that is affecting the Y-offset. 
I've tried changing the layer, script-priority, outside-staff-priority and just 
cannot get them level.

Is there any way to make the systems left and right align with the same value?

(I'm using Frescobaldi Music>Two pages (first page left) so you can see they 
are not level)

\version "2.20.0"

mymusic = { \repeat unfold 16 {c4} \break }

\bookpart {
\relative c' { 
 \overrideProperty Score.NonMusicalPaperColumn.
 line-break-system-details  #'((Y-offset . 40)))
    \mymusic }
}

\bookpart {
\relative c''' {
   \overrideProperty Score.NonMusicalPaperColumn.
   line-break-system-details #'((Y-offset . 40)))
 \ottava #1 
 \mymusic }
  }