The string idea is a good one, wish I had thought of that.

I balanced mine by the finger tip method. I put two marks on each wing, one mark at the forward CG and one mark reward CG (per instruction manual), and balanced (finger tip) just past past mid point closer to the reward mark. The plane flies great at this location, however I think it will do fine within the range given in the instructions, does not seem to be pitch sensitive at all. In fact I have been thinking of moving the CG backward.