I have (probably) survived numerous incidents (accidents) using this simple figure...
100 kg per passenger.
That includes the passenger (his/her fat belly if any), his/her checked baggage, carry-on, and duty free.
xxx
If given "347 pax" from the agent, on MY paper, I write 34,700 kg total payload.
If I am given 330 adults, 15 children, 2 infants, it becomes -
Adults 33,000 kg, children 750 kg, infants "0"... = 33,750 kg total.
I do not care what the official standard weights are. This is what I use for my takeoff computations and speeds.
So far, no chief pilots have fired me because I disregarded standard figures.
xxx
I am still alive after many years of flying - Retiring in 3 months (and happy)...
And in my 747, in does not matter where they sit. The CG is always OK.
No need to say, I have a B.Sc in Math/Physics... and I use that "superior" knowledge.
Takes me 1 minute to get my payload. F/Os or F/Es take 5 minutes, and arrive to... 33,336 kg.
And a lot of wear with their erasers ("rubbers" in UK, and territories).
xxx
Happy contrails
P.S. For Scanair departure from LEMD - I would assume their actual payload was 16,500 kg (165 adults?) -