  Majority of ultraluminous X-ray sources (ULXs) are believed to be
super-Eddington objects, providing a nearby prototype for studying an accretion
in super-critical regime. In this work, we present the study of time-lag
spectra of the ULX NGC 5408 X-1 using a reverberation mapping technique. The
time-lag data were binned using two different methods: time averaged-based and
luminosity-based spectral bins. These spectra were fitted using two proposed
geometric models: single and multiple photon scattering models. While both
models similarly assume that a fraction of hard photons emitted from inner
accretion disc could be down-scattered with the super-Eddington outflowing wind
becoming lagged, soft photons, they are different by the number that the hard
photons scattering with the wind: i.e. single vs multiple times. In case of
averaged spectrum, both models consistently constrained the mass of ULX in the
range of $\sim$80-500 M$_{\rm \odot}$. However, for the modelling results from
the luminosity based spectra, the confidence interval of the BH mass is
significantly improved and is constrained to the range of $\sim$75-90 M$_{\rm
\odot}$. In addition, the models suggest that the wind geometry is extended in
which the photons could down-scatter with the wind at the distance of
$\sim$10$^{4}$ - 10$^{6}$ $r_{\rm g}$. The results also suggest the variability
of the lag spectra as a function of ULX luminosity, but the clear trend of
changing accretion disc geometry with the spectral variability is not observed.
