ok let me explain this one more time.

ballance the arrow on a nail with the point installed, mark the ballancing point.

measure the arrow from the throat of the nock to the insert, this is the cut length of the arrow.

divide the cut length by 2, this will give you the center of the cut lengrh of the arrow.

mark the center and measure from the center mark to the mark of the balancing point.

multiply this measurement by 100.

then divide this number by the cut length of the arrow, this is your foc%

it is best to convert fractions to decimals.
in example, if you have a fraction of 1/4" simply divide 1 divided by4 this will give you the decimal equivelant.