When calculating stellar models,
the near-surface properties of convection are normally
treated with the mixing-length theory (MLT).
Here the parameter 
determines the jump in entropy from the
surface to the very adiabatic bulk of the convection zone,
thus hiding most of the complications of the near-surface region.
For the case of the sun, the value 
of can be found
by calibrating models at the present age to the solar radius. This is not
possible for other stars; thus it has become customary to assume
for all stars.
The work presented here is an attempt to change this, by
means of calibration to
realistic, numerical 3D-simulations of the surface region.