  Many unified models predict two large neutrino mixing angles, with the
charged lepton mixing angles being small and quark-like, and the neutrino
masses being hierarchical. Assuming this, we present simple approximate
analytic formulae giving the lepton mixing angles in terms of the underlying
high energy neutrino mixing angles together with small perturbations due to
both charged lepton corrections and renormalisation group (RG) effects,
including also the effects of third family canonical normalization (CN). We
apply the perturbative formulae to the ubiquitous case of tri-bimaximal
neutrino mixing at the unification scale, in order to predict the theoretical
corrections to mixing angle predictions and sum rule relations, and give a
general discussion of all limiting cases.
