I am pretty sure that the method to find a good starting CG for a bipe has been discussed here in the past. You should search for it and see if you can find it.

In short, find the mean aerodynamic cord of the wings and set the CG to be 25% of that to start. The MAC of a positive stagger bipe is the distance from the LE of the top wing to the TE of the bottom wing. Then you can adjust the CG via flight testing to the point that best suits your flying style.