I've said this before and it bears saying again. When in doubt, cut it out, of stiff paper. Cut out a scale overhead planeform of your ship, fold it slightly in the center to give it some stability and if necesary, apply a thin dowel stick to the centerline and add weight where needed to get the airplane to glide nicely. The plane shouldn't pump, and it shouldn't dive to the ground. Instead your looking for a nice flat glide. That's the forward CG of your airplane.