A numerical method to compute cometary dust tail brightnesses has been developed. The method, analogous in many ways to the inverse Monte Carlo method proposed by Fulle (cite{Fulle89}), has been applied to red continuum images of comet C/1999 T1 McNaught-Hartley obtained by Lara et al. (cite{Lara}). The time and size dependence of the particle terminal velocities, the size distribution functions, and the dependence with time of the power index of the size distribution have been derived. The best fits to the observed isophote fields are obtained for highly anisotropic dust grain ejection velocities. The power indexes of the time-dependent dust size distributions show small values, particularly shortly after perihelion is reached, in which a minimum of -5.5 in the power index of the size distribution function is found, suggesting the apparition of small particles. Our results are in line with previous results by Fulle (cite{Fulle92}) relating a systematic decrease of size distribution power index and dust ejection isotropy as the comet age increases. We also derived the dust mass loss rates and the Afrho quantity as a function of time, providing evidence that these two quantities do not have to be correlated if the size distribution and the dust ejection velocity are both time-dependent, in accordance with the results by Fulle (cite{Fulle2000}) for comet 46/P Wirtanen.