  Studies of the stellar and accretion properties of classical T Tauri stars
(CTTS) require comparison with photospheric spectral templates. Here we aim at
expanding the currently available grid of wide-wavelength coverage observed
spectra of non-accreting stars with additional new spectra and an interpolation
method that allows us to obtain a continuous grid of low resolution spectra
ranging from spectral type G8 to M9.5, while also mitigating observational
uncertainties. This interpolated grid is then implemented in the
self-consistent method to derive stellar and accretion properties of CTTS. With
the new templates, we aim to estimate a lower limit on the accretion
luminosities that can be obtained through a study of the UV excess emission
using observed templates. We analyse the molecular photospheric features
present in the VLT/X-Shooter spectra of the targets to perform a spectral
classification, including estimates of their extinction. We apply a
non-parametric fitting method to the full grid of observed templates to obtain
an interpolated grid of templates. We use the uncertainties on our interpolated
grid to estimate a lower limit on the accretion luminosity that we can measure
with this method. We find that the measurable accretion luminosities ranges
from $\sim 2.7$ dex lower than the stellar luminosity in M5.5 stars to $\sim
1.3$ dex lower for G8 stars. For young stars with masses of $\sim 1M_{\odot}$
and ages of 3-6 Myr this limit translates into an observational limit of mass
accretion rate on the order of $10^{-10} \rm M_{\odot}/yr$. The implementation
of an interpolated grid of observed templates allows us to better disentangle
degenerate solutions, leading to a more reliable estimate of accretion rates in
young accreting stars.
