I think it's actually worked out as (X-1)/5, not X/5. So the Rules page would seem to be wrong, compared to how I have always understood it working.
So, to put it in verbage, you gain half of your original Force probability every 5 levels. (And then the server runs a random check with this new value, to see if you are FS, and stores the appropriate values in the database if you are).
Maybe someone can verify it in the code.
"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."