There have been a couple of articles back in the 50's about free flight stability. I did some figuring comparing tail volume with CG position and came to the conclusion that the difference between a super stable Ramrod and a marginal Civy Boy amounted to a range of about 10% of MAC for the CG. Stable at the anterior position and marginal at the rear position. One can play with tail volume numbers and move this 10% range about anywhere you want, even a couple of inches behind the wing.

I went out one day with a free flight and a roll of solder and made flights with the CG moved around. With the CG forward the airplane would try to loop. I gradually moved the CG back. As it moved back the airplane took a shallower and shallower angle of climb. I actually got it to make a shallow dive on the last try. Fortunately it was a short engine run and I did not lose the airplane.