Don't use a header tank with a separate vent, no matter what
the height of the vent. There have been homebuilt crashes resulting
from unbalanced venting of tanks, and certified aircraft having
interconnected tanks (a "both" position) MUST have a common vent
system. The Cessna 172, for example, has a single vent unde the left
wing that is plumbed into the left tank, and a line from the top of
that tank to the top of the right tank. This maintains equal pressure
in both tanks, and therefore equal head on the fuel.
I recall a homebuilt that suffered engine failure because the
guy had installed a header tank under the panel fed by the wing tanks.
The header had its own vent plumbed up to the wing root, and due to
aerodynamic considerations had a slightly higher pressure than the wing
tanks. (It can easily happen when the wing tank caps are vented;
remember that the zone above the wing has a rather low pressure.) The
wing tanks would not drain into the header and the engine quit at a
really bad time after takeoff. The header should have been vented into
the top of the wing tanks.
Another case: the Glastar had (might still have) two wing tanks
plumbed into a single on-off valve in the cockpit. The tank vents were
run from each tank out to the tip of each wing, where they stuck down
into the slipstream and were cut on a 45 degree angle facing forward.
Besides scratching the head of anyone passing under the tip, they
provided uneven pressure to the tanks and one tank would run dry before
the other. If the pressure differential was large enough, the full tank
would not feed at all. We fixed that by running another line between
the inboard ends of the tanks. There were fittings conveniently welded
into the tanks at the right spot for this. The other drawback of tip
vents: Parked on a bit of a sideslope, the full tanks will send fuel
out the low vent, and the fuel from the high tank runs through the
plumbing (even with the valve off) and into the low tank, and you come
to work to find 15 gallons of fuel on the floor waiting to ignite.
We disconnected the tip vents and put ram tubes on the filler
caps. Stuck them up high enough to get out of the lowest pressure.
Gee, I'm talkative tonight. But I wish there was a website or a
book published for homebuilders with all the "Don't Do This" stuff in
it to keep us from making the same mistakes our predecessors made. It's
dumb to die twice for the same error.

Dan