The UDF example is to show how the parallel parts are put together. The face loops etc are covered earlier in the manual. Write the basic code and test on one core. Then look into adding the RP_HOST calls based on that part of the documentation. 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.
If done correctly numbers will match. I'd not use a UDF to find pressure drop, that's what the reporting functions are for. Finding a surface pressure to do "something" is what UDFs are for.