Numerical convergence of the penetration distance α for 2.5D and 3D simulations with different boost factors b. The radial grid spacing is expressed as the inverse number of radial grid cells, , so that first-order convergence corresponds to a straight line. The error bars indicate 3σ intervals of statistical variation. The grey shading shows σ, 2σ, and 3σ statistical-variation intervals of the fits. The dashed lines show the mean fit in each case and the dotted lines extrapolate the fits towards data points excluded from the fitting procedure.