mark the balancing point of the arrow
measure the arrow from the throat of the nock to the end of insert, this is your cut length.
devide the cut length by 2 to get physical center.
mark the center and measure the distance of the balancing point to the center of shaft.
multiply this measurement by 100
then divide that number by the cut length of the arrow
this will give you the foc %

do not try to determine foc% by components because it is very inaccurate method