MOTM

> I'm (finally!) just about to mount my first flat rails. Since the mounting
wholes on the front panels are bigger than the screw's diameter there's some
play when mouting.

Yes, this is part of Paul's design slop to allow for fit.  The same is true
on the 19" rack rails.

> When mounting several flat rails in a row, what's the nominal gap between
the rails?

When butting the rails end to end, there is zero gap.  They are a full 6U
wide.

> And is the upper/lower edge of the rails flush with the top/bottom edge of
the panels?

That depends on the cabinet.  In theory, the rails should be flush or up to
0.0075 higher (top) or lower (bottom) than the panels.  I have found the top
row of my slant cabinets you cannot put them all the way out against the
wood.  The truth you will find is that the measurements in your wood
cabinets cannot be near as accurate as those in metal.  Therefore, you can
adjust the flat rails somewhat as needed to compensate.

> My intention is to use one 2U and two 1U panels as templates, ie.
assembling panels and rails before mounting the rails.

This is the perfect plan.  You can leave the screws loose so when you get
your flat rails where you want them, the panel should still "rattle around"
on the loose screws so that you know the fit is good for all panels.

The best way is to use two 1U panels in positions 2 and 5.  If you look at
the flat rail wood screw holes, you will see you can still get to all 4 of
them with 1U panels in positions 2 and 5.  That was no accident.  :-)

Here are some photos of me installing flat rails in a dot com cabinet.  Some
of the issues with spacing and positioning are discussed.
http://www.wiseguysynth.com/larry/cabinet/dotcom/dotcom.htm

Larry