The CG placement is dependent on the desired pitch stability. The stability is determined by the distance the CG is ahead of the neutral point. A trainer should have a generous amount of stability but an aerobatic model should have neutral stability. Neutral stability is obtained by putting the CG at the neutral point. A generous amount of stability can be obtained by putting the CG 5 to10% of the mean aerodynamic chord ahead of the neutral point. A neutrally stable plane or an unstable plane does not have a trimmed flight speed and pitch attitude to return to. The neutral point is also the aerodynamic center of the whole aircraft. In the case of a pure flying wing the neutral point is the aerodynamic center of the wing. The aerodynamic center of the wing is at about 25%of the mean aerodanamic chord of the wing. The mean aerodynamic chord is the chord through the centroid of the area of one side of the wing.

With the aid of a gyro active stability can be obtained with the CG at the neutral point or even a little behind the neutral point as in the case of the full scale B2. The less wing area that is devoted to stabilizing and trimming the plane, the more wing will be available to produce lift and the less will produce drag. The more aft the CG, the more efficient the flying wing. This makes for an interesting trade off between stability and efficiency.

Everything I've said so far is based on the assumption that the pitch trim is not so bad as to put the plane into a stall. In the case of flying wings the pitch trim is built into the wing by the reflex in the airfoil, the blending of airfoils along the span and the washout in the wing. In effect, much of the pitch trim is designed and built into the airframe, which makes trim adjustments more difficult in most cases.

See:
http://beadec1.ea.bs.dlr.de/Airfoils/flywing1.htm

and

www.nurflugel.com