  We present and analyse quasi-simultaneous multi-epoch spectral energy
distributions (SEDs) of seven reverberation-mapped AGNs for which accurate
black hole mass measurements and suitable archival data are available from the
'AGN Watch' monitoring programs. We explore the potential of optical-UV and
X-ray data, obtained within 2 days, to provide more accurate SED-based
measurements of individual AGN and quantify the impact of source variability on
key measurements typically used to characterise the black hole accretion
process plus on bolometric correction factors at 5100 {\AA}, 1350 {\AA} and for
the 2-10 keV X-ray band, respectively. The largest SED changes occur on long
timescales (>1 year). For our small sample, the 1 micron to 10 keV integrated
accretion luminosity typically changes by 10% on short time-scales (over 20
days), by ~30% over a year, but can change by 100% or more for individual AGN.
The EUV gap is the most uncertain part of the intrinsic SED, introducing a ~25%
uncertainty in the accretion-induced luminosity, relative to the model
independent interpolation method that we adopt. That aside, our analysis shows
that the uncertainty in the accretion-induced luminosity, the Eddington
luminosity ratio and the bolometric correction factors can be reduced (by a
factor of two or more) by use of SEDs built from data obtained within 20 days.
However, mass accretion rate and mass accretion efficiency are mostly limited
by the unknown EUV emission and the unknown details of the central engine and
our aspect angle.
