A heuristic approach to investigate the characteristics of the numerical waves propagating in various CFD-based NWT set ups is proposed to develop an optimal grid system for a numerical wave tank. Linear dispersion relations of the waves in a Cartesian grid system are derived analytically. The analytic results lead to an optimal grid aspect ratio for the best dispersion approximation. Extensions of the analytic approach to determine an optimal set up of the grid system of CFD-based NWT are discussed. A roadmap to develop CFD modeling practices based on these heuristic approaches and further numerical verification is proposed for the on-going industry efforts to develop the guidelines for numerical wave tanks, such as the Joint Industry Project on “Reproducible CFD Modeling Practices for Offshore Applications”.