  The remaining theoretical uncertainties from unknown higher-order corrections
in the prediction for the light Higgs-boson mass of the MSSM are estimated. The
uncertainties associated with three different approaches that are implemented
in the publicly available code FeynHiggs are compared: the fixed-order
diagrammatic approach, suitable for low SUSY scales, the effective field theory
(EFT) approach, suitable for high SUSY scales, and the hybrid approach which
combines the fixed-order and the EFT approaches. It is demonstrated for a
simple single-scale scenario that the result based on the hybrid approach
yields a precise prediction for low, intermediate and high SUSY scales with a
theoretical uncertainty of up to $\sim 1.5$ GeV for large stop mixing and $\sim
0.5$ GeV for small stop mixing. The uncertainty estimate of the hybrid
calculation approaches the uncertainty estimate of the fixed-order result for
low SUSY scales and the uncertainty estimate of the EFT approach for high SUSY
scales, while for intermediate scales it is reduced compared to both of the
individual results. The estimate of the theoretical uncertainty is also
investigated in scenarios with more than one mass scale. A significantly
enhanced uncertainty is found in scenarios where the gluino is substantially
heavier than the scalar top quarks. The uncertainty estimate presented in this
paper will be part of the public code FeynHiggs.
