Normal Derivative values at boundary

Dear All

I am trying to get the normal derivative value at a specified boundary.

for this I have written code as:

real Nu  = int1d(Th, hot)( dx(T)*N.x + dy(T)*N.y); 

for( int n1 = 0; n1<=20 ; n1++)
{  court<< Nu(20/n1, 0) << endl;

Am I right for Nu values at a boundary hot.