Comment 2 for bug 1872594

In gnome shell:

Logo is aligned to 1 vertically, and then has margin of 3em

this._logoBin = new St.Widget({
    style_class: 'login-dialog-logo-bin',
    x_align: Clutter.ActorAlign.CENTER,
    y_align: Clutter.ActorAlign.END,
});

.login-dialog-logo-bin {
  margin: 3em 0; }

Where as in plymouth the Watermark has vertical alignment of

themes/bgrt/bgrt.plymouth.desktop:WatermarkVerticalAlignment=.96
view->watermark_area.y = screen_height * plugin->watermark_vertical_alignment - ply_image_get_height (plugin->watermark_image) * plugin->watermark_vertical_alignment;

Which means the margin in plymouth is 4% of screen height.

So one way to fix this is to make plymouth know what "3em" is and calculate that.
Or to teach gnome-shell to use 4% margin of the total screen.
I am going to try to do the latter.