I wrote about this extensively.
But in general, the recap is: if you get a Very Large aircraft full with cargo (especially the biggest ones - MD-11F, 777F, 747F), the margins are out of this world.
Margins of 757F are far more modest in comparison, not much better than a full passenger plane (after some adjustments that apparently lowered the default prices of cargo).
As far as HKG, it is an outlier in late stages of the game, as far as how much cargo demand there is.