Ferry,
Your FAC is your total number of penalties divided by the number of flights i.e.:
You Perfom 10 flight with 100 penalties each, your total penalty score is 1000 divided by the 10 flights therefore your FAC is 100.
You then perform a further 10 flights with no penalties giving a total penalty score of 1000 divided by 20 flights therefore your FAC is 50.
Hope that clears it up for you.