The most precise top quark mass measurements use kinematic reconstruction methods, determining the top mass parameter of a Monte Carlo event generator, $m_t^{\rm MC}$. Due to the complicated interplay of hadronization and parton shower dynamics in Monte Carlo event generators, relating $m_t^{\rm MC}$ to field theory masses is a non-trivial task. In this talk we present a calibration procedure to determine this relation using hadron level QCD predictions for 2-Jettiness in $e^+e^-$ annihilation, an observable which has kinematic top mass sensitivity and has a close relation to the invariant mass of the particles coming from the top decay. The theoretical ingredients of the QCD prediction are explained. Fitting $e^+e^-$ 2-Jettiness calculations at NLL/NNLL order to Pythia 8.205, we find that $m_t^{\rm MC}$ agrees with the MSR mass at the scale $1\,$GeV within uncertainties, $m_t^{\rm MC}\simeq m_{t,1\,{\rm GeV}}^{\rm MSR}$, but differs from the pole mass by $900$/$600\,$MeV.