Measure at 1/3 the mean aerodynamic chord; take a point on the centerline even with the upper wing L.E. and a point on the centerline at the lower wing T.E., find 1/3 of the distance from the front mark to the rear mark and transpose it to the underside of the top wing, and balance it there. That should get you in range, you can start from there. Works on every bipe I've ever flown. I think that Cox misprinted something; I agree with gboulton, I've never seen the c/g measured on the bottom wing of a bipe.

Try the point that cox recommends, and hold the plane upside down when you balance it, see if that works. If it does, try my method and see how close the two marks are. Move the c/g forward for more stability, aft for more agility (within a range of about 1/2 inch or so)