Seat load factor or weight load factor?
Seat load factor, in my book at least, is Revenue Passenger Kilometres (RPK) over Available Seat Kilometres (ASK) times 100. RPK is the number of revenue passengers multiplied by the distance each one flies. Revenue passengers include everyone except infants not occupying a seat, and staff pax regardless of their level of discount. Passengers flying free on frequent flyer redemptions do count. ASK is based on saleable seats, not necessarily the number physically fitted. Distances are Great Circle, between the airports involved.
Weight load factor is Revenue Tonne-Km (RTK) divided by Available Tonne-Km. That can be rather more complicated to calculate - but it's up to the airline to come up with how it determines passenger weights.
Doing it this way, with distance as part of the equation, means that you can calculate l/f for a single sector, for a multi-sector flight or across a whole network.