  We propose improved estimators to compute the reweighting factors which are
needed for lattice QCD calculations that rely on twisted-mass reweighting for
the light quark contribution and the Rational Hybrid Monte Carlo (RHMC)
algorithm for non-degenerate quark masses. This is the case for a number of
modern large-scale simulations based on O(a) improved Wilson fermions. We find
a significant reduction of uncertainties for the reweighting factors at similar
computational cost compared to the conventional estimation. This leads to a
significant increase in precision for phenomenologically relevant observables
with high correlation to the low eigenmodes of the Wilson-Dirac operator in the
presence of exceptionally small eigenvalues. Supplementary details regarding
the spectral gap of the light quark Dirac operator on the 2+1 flavor
large-volume ensembles explored in this study can be found in an accompanying
appendix.
