FOC is a measurable % of arrow`s center of weight. Over 20% can be ok in some situations, but below 7% can really give you problems. http://www.goldtip.com/calculatorcontent.aspx?coid=3
Seems like a center of gravity measurement off of the front of the arrow would make more sense to me. Example the cog should fall within the first 1/3 of the arrow