Never seen it done like that Arthur but it works. Another way of doing it is you do the first steps as Arthur stated . Measure what the difference from your balance point to arrow center, Take that dimensions times 100 and then divide that number by the total length of your arrow. We will use Arthurs numbers . 32 inch arrow halfway is 16 balance point is 4 inches from center. 4 times 100 = 400
400 divided by 32(total length of arrow) =12.5 % FOC