RE: Playboy
I have built many pylon designs that balance between 75 and 80% back from the LE. Granted, they all have large lifting (airfoiled) stabs.
I have started work on a Paul Gilliam Civy Boy that balances at 100%. That is correct; it balances on the very rear of the trailing edge. I have seen designs that actually balance around 110 to 115% back. You have to balance them upside down supporting the fuselage.
REMEMBER: with old timer designs, the balance point is a product of BOTH lifting surfaces, the wing and the stab.
I currently have a Playboy Jr. that flies great at 70%. Playboys have somewhat smaller stabs, so 65 to 70% makes more sense, but with the stab set at 0° and about 2 1/2° positive for the wing, they will fly fine at up to 80%.
The best thing to do is test glide the model over tall grass with the elevator centered. Adjust the balance until it glides with just a hint of a stall ( a slightly discernable porpoising). Don't use elevator trim, use weight. With fuel in the tank the glide will then be right on. You'll probably find that the balance is close to 70% when you achieve this.