More factors.....
TAAMGuy
Simplified: You also need to understand that although your friendly Oceanic Controller (through a Radio Operator) may have told you that the jets are at their final level after release from NY, that may only be the final NAT OTS level, due to traffic in front, above, behind and below. All a/c using the NAT OTS are separated procedurally (in lateral, stream and vertical plane) If some other guy is 6 minutes ahead and 1000' above you and you want to climb 1 level (1000'), it ain't gonna happen unless he's faster and until the controller can assure 10 minutes streamed separation. Level changes are reasonably common, depending on traffic. Flights coming off the Atlantic and going further than Dublin/Glasgow/Manch etc will probably be requesting a climb towards optimum FL for the remainder of the trip.
All of the above comments re: FMS optimum levels/Authority requirements/buffet margins etc are all taken into account too.
Ball park: B737-800 using default 5% CRZ CG at about 70000KG gives you an increase in optimum altitude of 100 feet every 9 (ish) mins, using 42 Kg/min fuel burn.