[css-ruby] how far from bases should ruby text be?

http://dev.w3.org/csswg/css-ruby/#ruby-position describes which side
of the base characters the ruby text should be on, but I don't see
anything there or elsewhere in the spec that describes how far from
the base characters the ruby text should be.  (There's a detailed
description of how the position, once known, affects line spacing,
but I'm asking what the position is.)

It seems to me that the spec should describe how the ruby text is
positioned relative to the bases (i.e., what metrics of both the
base and the ruby text are used) and whether properties like
vertical-align have any effect on this positioning.  If some things
are undefined because we don't know the right thing yet, that should
be stated explicitly, along with the constraints on the answer that
we do know.

-David

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                          https://www.mozilla.org/   𝄂
             Before I built a wall I'd ask to know
             What I was walling in or walling out,
             And to whom I was like to give offense.
               - Robert Frost, Mending Wall (1914)