There is methods and there is madness and there is chaos, depending on the operator.
Now some have changed to a system where the commercial flight number (the one on the ticket) is nowwhere near the one on the OFP, others distribute the OFP flight numbers by randon.
In certain cases you must use the same flight number in the OFP than in the commercial, since the traffic rights demand this.
Generally carriers divide their flight numbers into traffic zones, e.g. flight to North America are in the 100-199, flight to south america 200-299 etc etc etc.
Every carrier has his own system, and some simply have no system.