  We present an improved model for excess variance spectra describing
ultra-fast outflows and successfully apply it to the luminous (L ~ 10^47 erg/s)
low-redshift (z = 0.184) quasar PDS 456. The model is able to account well for
the broadening of the spike-like features of these outflows in the excess
variance spectrum of PDS 456, by considering two effects: a correlation between
the outflow velocity and the logarithmic X-ray flux and intrinsic Doppler
broadening with v_int = 10^4 km/s. The models were generated by calculating the
fractional excess variance of count spectra from a Monte Carlo simulation. We
find evidence that the outflow in PDS 456 is structured, i.e., that there exist
two or more layers with outflow velocities 0.27-0.30 c, 0.41-0.49 c, and
0.15-0.20 c for a possible third layer, which agrees well with the literature.
We discuss the prospects of generally applicable models for excess variance
spectra for detecting ultra-fast outflows and investigating their structure. We
provide an estimate for the strength of the correlation between the outflow
velocity and the logarithmic X-ray flux and investigate its validity.
