  Correlations between light neutrino observables are arguably the strongest
predictions of lepton avour models based on (discrete) symmetries, except for
the very few cases which unambiguously predict the full set of leptonic mixing
angles. A subclass of these correlations are neutrino mass sum rules, which
connect the three (complex) light neutrino mass eigenvalues among each other.
This connection constrains both the light neutrino mass scale and the Majorana
phases, so that mass sum rules generically lead to a nonzero value of the
lightest neutrino mass and to distinct predictions for the e ective mass probed
in neutrinoless double beta decay. However, in nearly all cases known, the
neutrino mass sum rules are not exact and receive corrections from various
sources. We introduce a formalism to handle these corrections perturbatively in
a model-independent manner, which overcomes issues present in earlier
approaches. Our ansatz allows us to quantify the modi cation of the predictions
derived from neutrino mass sum rules. We show that, in most cases, the
predictions are fairly stable: while small quantitative changes can appear,
they are generally rather mild. We therefore establish the predictivity of
neutrino mass sum rules on a level far more general than previously known.
