I do not understand this statement.
Then test by forcing a partition that cuts the surface(s) of interest: don't rely on Fluent's automatic method as it's intended to maximise efficiency gains. 
what do you mean by intended to maximize efficiency gains?
my question is, i will use the UDF in Section 7.8 on a simple fully developed laminar flow in a pipe problem as described above.
I will set int surface_thread_id=3; (in the second line of the udf printed, this is the thread ID of the inlet) 
and when the second to last line is executed,
This one:
Message("Average pressure on Surface %d is %f (Pa)\n",surface_thread_id,(total_force/total_area));
will this print/display on the console the average pressure value which is equal to the area-weighted average of pressure (calculated from surface integrals)?
please let me know.