I have just started to look into the FOC for my setup. It only makes sense to me that you measure the entire arrow tip of point to end of nock. This is the entire weight of the arrow. If you exclude the broadhead or field point you will get the FOC of the arrow without a tip and I would never shoot my arrow that way.
This is why the FOC can change if you switch from a 100 grain bh to a 125 grain.
Maybe I am missing something but that is the only way the FOC makes any sense to me. Hope this made sense and helped.