In this paper we examine the methodology of obtaining Cepheid-based distanc
es to galaxies from image data obtained with the Wide Field Planetary Camer
a (WFPC2) on the Hubble Space Telescope. We identify the sources of error a
nd how these propagate to the derived distances for the particular method u
tilizing the modified DoPHOT program for photometry. Typical uncertainties
from each of the identified sources are used to estimate the net uncertaint
ies. In particular, the effect of skewed error distributions from object co
nfusion and surface brightness fluctuation are studied, and tests to estima
te the extent of such skewness in real data are shown. The effect of charge
-transfer efficiency problems with the WFPC2 CCDs on photometric zero point
s is calibrated empirically. The corrections applicable to published data a
re given, which suggest that dereddened distance moduli using the DoPHOT me
thodology published to date have been underestimated by up to 0.1 mag.