There are a lot of works following the Neugebauer's study on modeling the reflectance of a halftone color. In the model, there are mainly two input parameter sets: one is the reflectance of primary colors, the ink solids and their overlaps; the other is the area of each primary colors, that is, the ‘fractional areas of Neugebauer primary colors’ (NPAs). For the later, Demichel's dot overlap law is widely used to estimate NPAs. However, the underline assumption of Demichel's law, that halftone dots of each ink are printed in a random arrangement, is not strictly true for most halftone patterns.In order to get more accurate estimation, a novel method is proposed to calculate NPAs by employing additional non Neugebauer primary color samples. It shows a significant improvement on estimating the reflectance of a halftone color. The average error (ΔE_ab*) is reduced from 4.7 to 1.82, and the maximum error from 11.53 to 6.55.