gtk_widget_size_request v. reality

[v4r4n <console cowboy gmail com>]

I believe this goes against the GTK philosophy, but for the sake of
backwards compatibility I'm trying to get the width of a hbox so that I can
calculate a hardcoded spacing percentage between two widgets and use the
number as pack padding (sorta like an invisible pane that the user can't
adjust).

How do I force a parent to tell the child how much space will be available
to the child at this time?  Is it even possible?  Do I really have to call
gtk_widget_set_size_request()/gtk_widget_size_request() on every parent so
that the children have something to work with?

I believe there is some kind of 'negotiation' between parents and children.
I basically want to know what the parent would give the child if the child
asked for more space than is available to the child (the max).

I'm new to GTK, but as far as I can tell, most GTK tutorials and
documentation don't really cover packing widgets when homogeneous, expand,
and fill are not all set to TRUE (if only I could just let GTK do it all for
me...).

Suggestions?

P.S. if this is the wrong email list, feel free to redirect me.

~Thanks