Calculation of alpha-particle confinement by a guiding center orbit-following numerical code requires the computation of very long particle trajectories. Due to their enormous length, these computations are subject to the possible accumulation of small errors, and the alpha-particle population is usually extrapolated from a single-particle history for every point of the initial parameter space. To overcome these difficulties, a numerical diffusion coefficient is derived for each point of the initial parameter space by averaging over a certain number of single-particle histories for each point of this space. This method has been applied to fast-alpha-particle confinement of the Next European Torus benchmark and the numerically derived diffusion coefficients are compared with analytical expressions from theoretical models.