It is done like the Davis cup, 100% of this years, 50% last years 30% two years ago, 20% three years ago.
But you then have to take into account if it was a friendly, the ranking of the opposition you were playing, their regional strength and average out the number of games.
An explanation is on the FIFA official site, I understand it in principle but without all the results, the ranking of the opposition when played and a lot of time, it has me beat.
I have never seen a site doing provisional rankings either. http://www.fifa.com/worldfootball/ranking/procedure/men.html