The altitude is also based on the direction in which the aircraft is flying.
In general, ICAO rules the following:
Below FL290, the altitudes for IFR aircraft go 2000/4000/6000/8000/10000 etc for 180-360 degrees, for 0-179 degrees it's 1000/3000/5000/7000/9000.
From FL290-FL410 Reduced Vertical Seperation Minima apply to approved aircraft. Most airliners are RVSM approved, so flight levels for these aircraft are Westbound FL300/FL320/FL340/FL360 etc, eastbound flights are FL290/FL310/FL330/FL350 etc.
Above FL290 for non-RVSM aircraft, 180-360 degrees is FL310/FL350/FL390/FL430 etc, and 0-179 degrees is FL290/FL330/FL370/FL410 etc.
The exceptions to the rule are as follows:
In France/Spain/Portugal, due the fact that most of the airways in these three countries are north-south directed the whole airspace structure for altitudes/FL’s are set to north-south as well. For northbound flights even FL
’s should be used and for southbound flights odd-flightlevels.
Over the North Atlantic, seperation is 1000 feet - so it goes FL310/FL320/Fl330 etc both ways - obviously dependant on other traffic.
Finally, in Russia altitudes are read in metric. All flights are leveled on meters. Vertical separation between two aircraft is 300 m from altitude 900 meters to altitude 8100 meters, 500 meters separation from altitude 8100 meters to altitude 12100 meters and a separation of 1000 meters from altitude 12100 meters to altitude 16100 meters.
Personally - I've cruised at everything from 10,000ft in a 737 to FL390 in a 744 - and everything in between - it really depends on the length of flight, the weather, fuel, heading and traffic so could be anything. I've heard 737NGs cruising at FL430 before overhead the UK.