  Massive star evolution is dominated by key physical processes such as mass
loss, convection and rotation, yet these effects are poorly constrained, even
on the main sequence. We utilise a detached, eclipsing binary HD166734 as a
testbed for single star evolution to calibrate new MESA stellar evolution
grids. We introduce a novel method of comparing theoretical models with
observations in the 'Mass-Luminosity Plane', as an equivalent to the HRD (see
Higgins & Vink, 2018). We reproduce stellar parameters and abundances of
HD166734 with enhanced overshooting (alpha=0.5), mass loss and rotational
mixing. When comparing the constraints of our testbed to the systematic grid of
models we find that a higher value of alpha = 0.5 (rather than 0.1) results in
a solution which is more likely to evolve to a neutron star than a black hole,
due to a lower value of the compactness parameter.
