What are the symptoms? If it is unstable in pitch then it is tail heavy. If it just needs down trim, give it some down trim. Also check your ailerons to make sure they are not a little bit down and functioning as flaps. That could cause a need for down trim.

I balanced mine at the rear of the suggested range and like it that way.

Jim

Edit: Re-reading it sounds like you haven't flown it yet and are just trying to get it to the specified CG. I balanced mine with an OS 70 FS, but I had lightened the tail some, and also got the battery well forward, under the tank, I think. I also used an aluminum spinner, which helped.

Don't fly it tail heavy. Use a heavy hub if you need to. Then you can carefully try moving the CG back, a little at a time, depending on how it flies.