Determine FOC:

To find the FOC (which is always expressed as a percentage) divide the overall length by two. This should produce the physical center of the shaft. Now subtract this number from the balance point and divide by the overall length. Multiply by 100 to express the fractional value as a percentage.

Most expert archers agree that an FOC value that is between 7 and 10 percent will produce the best compromise between stability and a flat trajectory. The American Society for Testing and Materials, in their specification for measuring balance point, state that a value of 9% is typical. But, they also state that the range can be as wide as 7% to 18% while still producing good arrow flight characteristics.