I remember a rule of thumb relating to this. It was to draw a line from the center of gravity forward at a 30 degree angle. The point where the line intersects the ground is where the main wheel should contact the ground. Further forward, and the airplane will tend to be squirrely, further aft and it will be more prone to nose over. A quick observation of a selection of taildragger aircraft will show this to be a fairly common set-up.

Remember, this is a general rule. Your mileage may vary.

-Robert