The calibration of color printers is highly influenced by optical scattering. Light scattered at microscopic level within printed papers induces a blurring phenomenon that affects the linearity of the tone reproduction curve. The induced nonlinearity is known as optical dotgain. Engeldrum and Pridham analyzed its impact on printing, using Oittinen's light scattering model. They determined the scattering and absorption coefficients based on spectral measurements of solid patches only. Their calibration achieves good independence of any printing irregularities. However, the microscopic knife-edge measurements of Arney et al showed that the model overestimates the influence of the absorption coefficient. Unlike Oittinen's model, we directly approach the laterally scattered light fluxes. This is achieved by an extended three-dimensional Kubelka-Munk model. We describe how to determine our coefficients using measurements of mere solid patches, which allows us to decouple the optical dot gain from other printing influences. Our improved model successfully corrects the observed overestimation and is able to predict Arney's microscopic measurements.