  We present a new physically-motivated parameterization, based on the ratio of
Thorpe and Ozmidov scales, for the irreversible turbulent flux coefficient
$\Gamma_{\mathcal M}= {\mathcal M}/\epsilon$, i.e. the ratio of the
irreversible rate ${\mathcal M}$ at which the background potential energy
increases in a stratified flow due to macroscopic motions to the dissipation
rate of turbulent kinetic energy. Our parameterization covers all three key
phases (crucially, in time) of a shear-induced stratified turbulence life
cycle: the initial, `hot' growing phase, the intermediate energetically forced
phase, and the final `cold' fossilization decaying phase. Covering all three
phases allows us to highlight the importance of the intermediate one, to which
we refer as the `Goldilocks' phase due to its apparently optimal (and so
neither too hot nor too cold, but just right) balance, in which energy transfer
from background shear to the turbulent mixing is most efficient.
$\Gamma_{\mathcal M}$ is close to 1/3 during this phase, which we demonstrate
appears to be related to an adjustment towards a critical or marginal
Richardson number for sustained turbulence $\sim 0.2-0.25$.
