F.O.C. = Front Of Center

Measure the length of your arrow from the front of the insert to the bottom of the notch in the nock.

Divide that by 2.

Find the balance point of your arrow.

Measure from the bottom of the nock to the balance point.

Subtract 1/2 arrow length from the measurement to the balance point.

Divide that by the arrow length

Multiply by 100 = FOC %

For instance, my arrow is 32" long.

1/2 that is 16"

Balance point measures 20" from the nock.

20-16 = 4

4/32 = 0.125

0.125 X 100 = 12.5

FOC on that arrow is 12.5%

Or go online, Google up 'Archery FOC calculator'and use one of the myriad FOC calculators out there. but expect different results with different calculators because not everyone actually agrees on how to measure FOC. Some folks insist the only proper way is to include the length of the point in the overall length measurement. The way I described is AMO standard.