I work for Jetstar a LCC in Australia. We have an open seating arrangement like Southwest in the states. Up until recently when policy changed, we would ensure the correct trim by blocking off designated rows of seats in the aircraft.
About a couple of months ago, this procedure has changed and now the correct trim is worked out by counting the pax in the forward and aft sections of the aircraft. If the differential is greater than 20 pax in either forward of aft, pax will be asked to move seats.
