  We study the theoretical uncertainties in the determination of the top-quark
mass using next-to-leading-order (NLO) generators, that describe the top-quark
decay at different levels of accuracy, interfaced to parton showers (PS).
Specifically we consider one generator that implements NLO corrections only in
the production dynamics, one that also takes them into account in the top-quark
decay in the narrow width approximation (NWA) and one that implements them
exactly, including finite-width and interference effects. We aim at assessing
the errors in top-mass determinations of purely theoretical origin. We do so by
measuring relative peak position shifts of $Wb$-jet mass distributions. Besides
the theoretical errors due to the use of less accurate NLO+PS generators, we
also explore uncertainties related to shower and modelling of non-perturbative
effects by comparing the results obtained by interfacing our generators to both
Pythia and Herwig shower Monte Carlos (SMCs).
