While not perfect, a definition of "Wing Volume Loading" proposed by a Mr. Reynolds who wrote for Model Builder, seems to work better than anything else I have tried. His formula is WingVolLoading = (weight of model in ounces) divided by (wing area in square feet raised to the 1.5 power). If your calculation comes out in the 7 to 10 area, you have a good chance of a flyable airplane. If the value is greater than 10, you are approaching a lead sled type vehicle. If less than 7 the plane becomes a real floater. Most trainers fall in the 6 to 8 range.