[Usability] How much intendation should be used for grouped widgets?

[Christian Neumair <chris gnome-de org>]

I originally sent this mail to the hig list, but as I'd like to reach a
bigger developer/subscriber base, this mail has been resent to the
usability list.

Our current HIGifying situation is more than unsatisfying when it comes
to the category intendation. Currently three different values are used:
12 px, 18 px as well as 4 spaces ("    ").
While the HIG states that you ought to "(i)ndent group members 12 pixels
to denote hierarchy and association." [1], another image on the same
page [2] obviously uses 18 px for that. In my opinion this shows that
the HIG authors themselves prefer 18 px over 12 px.
In my AbiWord HIGifying efforts I used 18 px for that intendation and I
must say that it looks way better than 12 px. Third candidate it the
"    " intendation, but I don't think it's a good idea to make the
indenation dependant on the currently used font width.

regs,
 Chris

[1]
http://developer.gnome.org/projects/gup/hig/1.0/layout.html#window-layout-spacing

[2]
http://developer.gnome.org/projects/gup/hig/1.0/images/layout_callouts.png