.... zero?

https://www.flickr.com/photos/gdguar...7644207411490/

I used a hollow-core door with a piece of mdf on top as a work surface.
I screwed down a long aluminum straight edge and a strip of 1/2" ply to
make a square corner. Then I screwed down four small blocks of 1x2 scrap
at a slight angle to the work piece, leaving about a 1/4" gap. The
blocks are placed adjacent to the four rails that I glued in, by the
way; the rest of the rails float. Four shims provide the clamping pressure.

I've glued up the first two "ladders" (of four) that form the sides of
my shelf units. It seems to be working fine. One of the reasons I did it
this way was that - at least in the past - I have had some challenges
getting stuff square. If I had to do any persuading (which would be
difficult with this geometry and all of the dowels), I'd know right away
and have a ready reference. I'm happy to report that the pieces went
together square without any problem, perhaps because I took more care to
set things up in the beginning.

I see no inherent problem with scaling up this system (which I certainly
did not invent), but you would need a very large flat surface (that you
don't mind driving screws into) for Leon's headboard

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