As a follow on, if (say) a 747 had a 90% passanger load, allowing for their luggage, approx what weight of freight could be included in the plane as "belly freight" ?
LH Cargo's website is worth a look.
At the bottom of
this page, you can download a pdf with all containers and LH Group aircraft types with payload/container mix etc.
747-400 has a belly payload of 14000k, averaged over the network, 747-200F operating out of FRA was weight limited as from NYC (unconstrained total payload 101.1t) dropping to 93.7t for ORD, 88.4t for ATL and 73-ish for MIA (fuel stop in BGR helped out there. Even better - YYT, direct on the track)