RE: Sadler Vampire from Nitro Models
On most straight winged planes anywhere in front of 25% of the chord is perfectly safe.
For swept wing and bi-planes you want to have the C.G. forward of 25% of the mean chord length.
Many of my 3D planes go back as far as 30-40% but the latter can be iffy.
With the Vampire it is almost difficult to get the C.G. too far back if the plane sits level on the ground when ready to fly.
For it to sit level the C.G. has to be in front of the wing landing gear area, which is in turn also in front of the 25% chord mark.