  In flipped SU(5) grand unified theories, the partial decay lifetimes of
certain nucleon decay channels depend generically on an unknown unitary matrix,
which arises when left-handed lepton fields are embedded into anti-fundamental
representations of SU(5). This dependency is particularly relevant when the
neutrino mass matrix has a generic structure, introducing uncertainty in the
prediction of nucleon decay branching fractions within flipped SU(5). In this
paper, we demonstrate that this uncertainty can be parametrized using two
parameters, which can be determined by measuring the partial lifetimes of $p
\to \pi^0 e^+$, $p \to \pi^0 \mu^+$, and $n \to \pi^0 \bar{\nu}$. In addition,
we establish upper limits on the ratios of the decay widths of these channels,
offering a potential method to test flipped SU(5) in future nucleon decay
experiments.
