I am using 3-4" Duravanes which weigh 8 grains a piece, for a total of 24 grains.
Switch to 4" feathers at 2.3 grains per each for a total of 6.9 grains and see what your FOC does.
Even the big 5 3/4" Magnum feathers are only 4.2 grains per each.
I don't agree or disagree with taking broadhead length into account. I understand the principle behind it but it really doesn't mean that much, IMO.