Many wings have their leading edges sheeted back to the main spar. Then it's open rib bays until the trailing edge sheeting. If your model is like this then use the rear edge of the leading edge sheeting as your CG point. If not -- measure the chord of the wing, the distance from the leading edge to the trailing edge, to determine the mean aerodynamic chord, MAC of the wing. Then set your CG 25% of that distance back from the leading edge. This will put your CG at 25% MAC and thats a good starting point for a constant chord wing. From there you can fine tune the CG with flight testing to get it where it best suits your flying style.