  We quantify and examine the uncertainties in predictions of the lightest $CP$
even Higgs boson pole mass $M_h$ in the Minimal Supersymmetric Standard Model
(MSSM), utilising current spectrum generators and including some three-loop
corrections. There are two broadly different approximations being used:
effective field theory (EFT) where an effective Standard Model (SM) is used
below a supersymmetric mass scale, and a fixed order calculation, where the
MSSM is matched to QCD$\times$QED at the electroweak scale. The uncertainties
on the $M_h$ prediction in each approach are broken down into logarithmic and
finite pieces. The inferred values of the stop mass parameters are sensitively
dependent upon the precision of the prediction for $M_h$. The fixed order
calculation appears to be more accurate below a supersymmetry (SUSY) mass scale
of $M_S \approx 1.2$ TeV, whereas above this scale, the EFT calculation is more
accurate. We also revisit the range of the lightest stop mass across fine-tuned
parameter space that has an appropriate stable vacuum and is compatible with
the lightest $CP$ even Higgs boson $h$ being identified with the one discovered
at the ATLAS and CMS experiments in 2012; we achieve a maximum value of $\sim
10^{11}$ GeV.
