That's what I've found too. I started out with my 1800mah pack and receiver right on the rearmost former in the radio compartment, and then started adding weight to the tail. To get to 6.25", it took 3.25 ounces. This is where I really like it, so last night I put it back on the CG machine, removed the weight and marked where I wanted to cut a hatch and mount the battery. It is really about halfway between the stab and the ailerons.
For Waterfalls, mix in up spoileron with your down elevator and it'll tighten right up. Up Spoileron for Harrier as well will calm the rocking, that combined with the rearward CG will all but stop it.
Mine has gotten easier and easier to hover as the CG has moved back, but I do have 2 washers under the left side of the motor mount to get in some right thrust, and it could probably use 3 truthfully, but my cowl lines up so good I don't want to mess that up.