The transport of charged particles in various astrophysical environments permeated by magnetic fields is described in terms of a diffusion process, which relies on diffusion-tensor parameters generally inferred from Monte-Carlo simulations. Based on a red-noise approximation to model the two-point correlation function of the magnetic field experienced by charged particles between two successive times, the diffusion-tensor coefficients were previously derived in the case of pure turbulence. In this contribution, the derivation is extended to the case of a mean field on top of the turbulence. In addition, the red-noise approximation is relaxed by modeling the relevant two-point correlation function obtained by Monte-Carlo simulations for a wide range of test-particle rigidities.