  We compare standard models of accretion disks around black holes that include
the appropriate zero-torque inner boundary condition and relativistic effects
on the emission and propagation of radiation. The comparison is performed
adopting the multicolor disk blackbody model (MCD) as reference and looking for
the parameter space in which it is in statistical agreement with "more
physical" accretion disk models. We find simple 'recipes' that can be used for
adjusting the estimates of the physical inner radius of the disk, the black
hole mass and the accretion rate inferred using the parameters of the MCD fits.
We applied these results to four ULXs for which MCD spectral fits of their
X-ray soft spectral components have been published and find that, in three
cases (NGC 1313 X-1, X-2 and M 81 X-9), the black hole masses inferred for a
standard disk around a Schwarzschild black hole are in the interval ~100-200
solar masses. Only if the black hole is maximally rotating are the masses
comparable to the much larger values previously derived in the literature.
