Specifying space-before relative to the page margins

[Xmplar <info@xmplar.biz>]

I have on a book title page several elements (title, subtitle, author, contributor, mediaobject etc) all centred below each other. For each element, I have set space-before values which are, of course, measured from the preceding element (so, the space-before for subtitle is applied relative to the position of the title).
Instead of specifying space-before for each element relative to the preceding element, is it possible to specify the space-before relative to the top margin of the page?

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Dave Gardiner