According to Ray Goldie's book, "Inco Comes to Labrador", the value of DE1 per share should be calculated on the basis of 70% of the Net Present Value (NPV), not 10% of the Net Smelter Royalty (NSR). The NSR value is used in the calculation of NPV. However, I'm not a mining analyst and can only assume that what he said is more or less common practice for valuing mining stocks amongst the investment community.