Anyone interested in this, have a look at

/home/td_d570_pro_data_tmp/N000042618807-IMSDALI-AC20201125-N/24X/tmp/fopostpro-tmp.dvi (airbus.com) - the range/payload/take off weight calculations are 140 et seq. There are standard estimates for passenger plus baggage weight, and fuel is also taken into account. You can find information on Florence here

LIRQ/Florence Amerigo Vespucci Peretola (Firenze) General Airport Information (acukwik.com)

It is indeed an extremely short runway at 5118 feet. As a point of comparison, Southend, which is also a short one, is just over 6000. Heathrow is 13000