Frank-Kamenetskii Rheology

Dear all,
I am trying to run some models using the Frank-Kamenetskii approximation for rheology and adiabatic temperature profile. I would like to have some information about the meaning of the bottom thermal boundary age. Could it be set just the top thermal boundary layer for the half-space cooling computation and leave the bottom as the default value (i.e., 0) to have a correct temperature from the lithosphere down to 150 km depth? Is the Constant viscosity pre-factor to be set to 1 (default value) to correctly relate with FK viscous flow law?

Thanks for your help,