Average of "Coefficient Form Boundary PDE" value

Hi all,

There are two coupled physics in my problem. I've used Coefficient Form Boundary PDE for finding the value of a variable in a boundary (call it P), and I need to use its average to solve the equation of domain for finding the value of another variable (call it J). I don't know how to do that! Can any one guide me pleae? The coupling of the physics is a two way coupling. The variable of the domain equation (J) is used in finding the variable of the boundary (P).

Thanks in advance