Staff Centre Vertical Alignment on page?

[Stefano Antonelli]

Hello List,

I'm producing muliple pngs with a single staff each.  When I 'flip'
through these pngs, the staff appears to move up and down on the page.

I can set

top-system-spacing.basic-distance = #7

in my \paper block and that seems to work for the one piece I'm playing
with, but it depends on what else is going on in the music.  I don't
have a good way to determine the best value.

If left at the default, beams will push the staff down on some pages,
for example.  Measure numbers also move it down from the first image
which doesn't have a measure number.

Basically what I want is for the staff lines to remain in the same
position no matter what's going on in the music.  If I could centre the
staff to the vertical centre of the page, that would be ideal.  I
didn't see this type of alignment in the manual.

The documentations says for ly:one-line-auto-height-breaking:

"Note that the top-system-spacing setting will affect the
vertical position of the music.  Set it to #f in a paper block
to simply place the music between the top and bottom margins."

Not quite sure that's the behaviour I want, but without ly:one-line-
auto-height-breaking it doesn't seem to do it anyway.

Is it possible to centre the staff to the vertical centre of the page?

Thanks,
Stef