#1. Multiply shaft grains/inch by finished arrow length - nock groove to other end of carbon.
#2. Multiply shaft grains/inch by the distance from nock groove to end of carbon at nock end. change to negative number.
Add weight of bushing, nock, fletch to the negative number above. this your net weight of nock, bushing, and fletch.
#3. Add #1 + #2 + point weight.
#4. Subtract #2 from PW, then divide that in half, then divide that by total arrow weight (#3).
This result is your FOC.