Last time I balanced a biplane, I attacked the problem a slightly different way. I marked the C of G range on the top wing, then decided where I wanted the C of G in that range. It was where the wing panels join so I drilled a 1/16 hole through the covering and glass join in the wing into the pair of ribs that formed the join and screwed in a cup hook. I then balanced the plane front to rear as well as laterally , el perfecto!
I then covered the hole with a sticker, you'd never know it was there, but its quite easy if you ever need to reckeck the balance.