  Uncertainties in the nuclear transition matrix elements $M^{(0\nu)}$ and
$M^{(0N)}$ of the double-positron emission $(\beta ^{+}\beta ^{+})_{0\nu}$ and
electron-positron conversion $(\varepsilon \beta ^{+})_{0\nu}$ modes due to the
exchange of light and heavy Majorana neutrinos, respectively, are calculated
for $^{96}$Ru, $^{102}$Pd, $^{106}$Cd, $^{124}$Xe, $^{130}$Ba and $^{156}$Dy
isotopes by employing the PHFB model with four different parameterization of
the pairing plus multipolar two-body interactions and three different
parameterizations of the Jastrow short range correlations. In all cases but for
$^{130}$Ba, the uncertainties are smaller than 14% for light Majorana neutrino
exchange and 35% for the exchange of a heavy Majorana neutrino.
