  Due to their high intrinsic brightness, type II supernovae (SN) can be used
as lighthouses to constrain distances in the Universe using variants of the
Baade-Wesselink method. Based on a large set of CMFGEN models (Hillier & Miller
1998) covering the photospheric phase of type II SN, we study the various
concepts entering one such technique, the Expanding Photosphere Method (EPM).
We compute correction factors $\xi$ needed to approximate the synthetic
Spectral Energy Distribution (SED) with that of a blackbody at temperature $T$.
Our $\xi$, although similar, are systematically greater, by $\sim 0.1$, than
the values obtained by Eastman et al. (1996). This translate into a systematic
enhancement of 10-20% in EPM-distances. We find that line emission and
absorption, not directly linked to color temperature variations, can
considerably alter the synthetic magnitude, and cause above-unity $\xi$-values.
Optically-thick lines, present in the optical at late-times, can also introduce
a strong wavelength-dependence of the photospheric radius, invalidating the use
of the Baade method. Both the impact of line-blanketing on the SED and the
photospheric radius at low $T$ suggest that the EPM is best used at early
times, when the outflow is fully ionized and line-opacity mostly confined to
the UV range. We also investigate how reliably one can measure the photospheric
velocity from P-Cygni line profiles. Contrary to the usually held belief, the
velocity at maximum absorption in the P-Cygni trough of optically-thick lines
can both {\it overestimate or underestimate} the photospheric velocity, with a
magnitude that depends on the SN outflow density gradient and the optical
thickness of the line.
