Six inches is way too far back. I built my DP edge (with an OS 1.6) and when I went to balance it (at 6 inches) the nose came down - way hard. I installed weights in the tail and mounted the battery as far back as I could.

This didn't seem right so instead of flying the plane the next morning I got on-line (here) and found several posts that the CG should be 5 inches and not 6 inches. I took out the weights out and it balanced to about 5.25". I took it out to our field and had a well known Unlimited IMAC pilot test fly the plane. He said it was still tail heavy and didn't think it would have been flyable with the CG at 6" (consistent with the other posts I read). I have since replaced the the OS 1.6 with a DA 50 (sweet) and the CG is at 4.75 inches. It flies well but is probably a little nose heavy.