OK, this one is REALLY only of interest to scripters, but the way the fWait setting gets used to determine script likelihood is as follows
there is a 1/fWait chance the script will be considered for inclusion in the current mission list, i.e. an (fWait-1)/fWait chance it will be discarded out of hand
So scripts with fWait of 1 are always examined, fWait 2 gives a 50% chance of being considered fWait3 gives a 33% chance of being considered, etc (no apparent upper bound)