Table-look-up modeling of microwave electron devices needs data interpolation algorithms with specific requirements. In the paper, an original data approximating formula is adopted in the framework of a well-known LUT-based nonlinear device model. Accurate intermodulation distortion prediction for highly linear power amplifier design is proven to be strongly dependent on the data interpolation/ approximation. The proposed procedure provides more accurate reconstruction of sampled functions and related derivatives than conventional splines. The LUT-based model is shown to provide accurate predictions of the third order intermodulation product even at extremely low levels.