Its about spine , and enough stearing or fletching in the rear of the arrow . Genraly when you try to go lite , you are to week on spine , and that kills fixed broadhead accuracy . Spine would be my first concern , enough of it and consistancy from arrow to arrow Throw some 5" feathers on there , and then worry about foc . If you are trying to save weight by going to a lite broadhead 75 grains or lighter , and you are useing 5 inch vanes you may have a problem down range in the wind because of foc . If yogo with feathers and a heavy broadhead your trajectory will look like a rainbow , and pin gap will increase , archery is a bunch of tradeoffs .