Fractional differences of the model covariance to the measured for the GRF, each row showing a different field size. In the left column are the fractional differences between the full analytic model and the simulated GRF. The middle column shows the difference between the large-field approximation, neglecting the finite field term, and the simulated covariance. In the right column, the difference of the model using the large-field approximation for T_PPP, 1, but keeping T_PPP, 2 as it is, is shown.