Fletched darts need only have a balance point that is 6 to 8 inches forward of center. The 2/3 thing applies to unfletched darts. An unfletched dart will fish tail through the air unless the balance point is 2/3 forward. Such a dart, having so much of its weight forward of center will be “point end heavy” and will tend to drop like a rock, need a higher trajectory of flight to get to the target. The purpose of the fletching is to make the dart’s flight more stable. The bigger the fletching, the more stable the flight, but, the bigger the fletching, the slower the flight, so, eventually, if the fletching is too big, you get the same result of having to have a higher trajectory to the target.
My fletching (feathers) are 10 inches long, by 1 1/2 inches wide. This gives me a flat trajectory flight at 15 meters, while I have to aim 6 inches above the bullseye at 20 meters, and 1 foot above the entire target at 25 meters. My darts are 6 feet, 10 inches long, and weigh about 6 ounces.