Micro-perforated panels (MPPs) have been widely used for broadband noise absorption in various engineering applications. When a MPP is exposed to a grazing flow, literature shows that existing acoustic impedance formulae based on different flow parameters give very different results. Therefore, there is a need for investigating the issue and finding more relevant parameters for the reliable acoustic impedance predictions. The issue is technically challenging because of the flow complexity near the hole of the panel. In the present work, 3D URANS CFD simulations are carried out to investigate the acoustic behaviour of a MPP hole with a backing space under grazing flow. Based on one-cell simulations and numerical experiments, the porosity of the panel is then introduced, leading to a new acoustic resistance formula. The proposed formula uses the velocity gradient in the viscous sublayer over the duct wall as the new parameter, and is found to be applicable at a Mach number up to roughly 0.25, within a certain Reynolds number range and under the linear acoustic excitation regime. The accuracy and the improvement of the model are demonstrated through comparisons with some existing ones as well as with the experimental data reported in the literature.