Great Planes made/makes the PT-40 series.
Unless you change the geometry of the wing, you should not deviate too far from the manufacturer's recomended balance point. On this plane a good starting point would be about 27-30% of the mean aerodynamic chord (MAC) for stability. Check around in the aerodynamics forum and they can help you out with understanding CG better.
By the way, in stock form this plane is very stable and will make a great primary trainer.
After you learn to fly it well, I had a freind who put a bigger engine on the nose, larger control surfaces, added flaps, and converted it to a tail dragger and made it into a well-behaved bush plane.

-Steve