Does anyone know of a good article / forum thread etc about "centre of gravity" and general plane set up for 3D?

The "best" way to do it is in the air.

Overall, for good 3D, it will be an aft CG. Hover the plane, good 3D balance is when in a hover, you do not have to work the elevator, normally pulling up elevator to keep the nose up. This is kinda difficult to discribe.

Another good way, is, when inverted, you want the plane to climb a bit.

Too tail heavy is when the plane balloons on landing and you need down elevator to land.

Start safe and move it back. Good flying 3D planes will fly very tail heavy. Bad flying 3D planes will snap.

Don't forget there is more to 3d then CG you will need to experiment with throws, expo, and engine thrust lines to really get it tuned in
But I would start with the CG like YNOT said I would work it back slowly and fly it more the once at each setting if it gets too pitch sensitive or snaps out you have crossed the sweet spot
I'm playing a Magic 3d right now and found that adding some spoilers has help keep her from falling to the belly in a hover
so don't be afraid to play with all the settings but do them one at a time and 3 mistakes high till you get it where you want it
good flying

Measure the chord of the wind multiply by .33 and then put the cg that far back from the LE.

Stick 40

That only applies to constant chords not tapers you can really mess up doing that

Opps my bad!

Stick 40

I start with the CG right at the wing spar and go from there. Also, dual rates are almost a must until you get everything trimmed out. For the first flight I would recommend low rate on the elevator and ailerons at between 30% and 40% depending on the size of the control surfaces.
As far as articles, there are countless threads on the subject. Just do a search for 3D setup. You might even try looking up the plane on the Mfgr's website and see if the manual is online.