I've been flying my UCD at 6.5" back. Engine is a ST 90 spinning an APC 15x4w. Fuel tank is in the stock location.

Until the tank is nearly empty, the plane still falls in hands off inverted flight......which indicates it's still actually nose heavy. When the engine is running on fumes, the UCD comes close to maintaining hands off level inverted flight, but it definitely doesn't climb.....so it still isn't too tail heavy. Deadstick landings with an empty tank require a light touch on the elevator when approaching and flaring for landing, but I don't consider it to be unstable.

With the fuel tank on the CG, I would aim for an initial CG of 6 3/8" back. And I'd be willing to bet that after flying the UCD for awhile, you'll settle on a CG that will be within 1/4" of this mark.