We revisit the mechanism of neutrinoless double beta (0νββ) decay mediated by the exchange with the heavy Majorana neutrino N of arbitrary mass m_N, slightly mixed ∼U_eN with the electron neutrino νe. By assuming the dominance of this mechanism, we update the well-known 0νββ-decay exclusion plot in the m_N − U_eN plane taking into account recent progress in the calculation of nuclear matrix elements within quasiparticle random phase approximation and improved experimental bounds on the 0νββ-decay half-life of 76Ge and 136Xe. We also consider the known formula approximating the m_N dependence of the 0νββ-decay nuclear matrix element in a simple explicit form. We analyze its accuracy and specify the corresponding parameters, allowing one to easily calculate the 0νββ-decay half-life for arbitrary m_N for all the experimentally interesting isotopes without resorting to real nuclear structure calculations.