Remove the point from your arrow, measure the length of the arrow from the bottom of the nock groove to the tip of the point insert and divide by 2. mark that as the center of your arrow.

Install the point, balance it precisely and mark that spot. It will be forward of your center mark.

Measure the distance between your center point and the balance point.

That distance divided by the arrow length equals the FOC as a percentage.

Example: 30 inch arrow with a 3 inch center to balance point equals a 10% FOC.

I have shot arrows with a 12% FOC that worked great.