surface average temperature

I want to model a concrete slab with a void inside the concrete. It is a thermal analyse where I apply a heat flux to the bottom surface. Within the void heat is transferred by radiation and convection. For simulating heat convection within the void, I use the surface film condition function in interaction module. I have to define the film coefficient and sink temperature. I decided to use the average temperature of the void surface for sink temperature. I don’t know how to define the average surface temperature. I think I have to use Abaqus Subroutine.