With all due respect, that's a bunch of hooey. You have to make informed choices on how you make up and arrow and what weight broadhead you choose or you can get in big trouble when it comes to tuning and sighting in.
A 28" 2216 with 5" vanes and a 75 gn head will be around 6.3%. A Beman ICSH 340 with a 75 gn head and 4" vanes - not an unusual arrow at all - FOC's at a mere 7.2%
are you determining FOC% by components or are you physically getting the balance point of the arrow and the center of the cut length?