hbox with "width"

[Dmitri Hrapof <hrapof common-lisp ru>]

Hello!

I'd like to insert two widgets into my top level window so that the first widget
always occupies, say, 70% of the total width. 
Like in HTML table with "width" attribute.
Is it possible to do this with Glade/hbox properties/Alignment etc. 
not resorting to "manual" width calculation at run-time?

Sincerely yours,
Dmitri