I balanced mine at the aft edge of the main spar and it seemed to not be too far back by any means. The distance on the plans shows more towards the front of the spar. According to Sig, you should start with the C/G 3 5/8" aft of the leading edge but they say they have flown the model with it as far aft as 3 7/8". You will have to balance the model upside down.

Enjoy,

Jim