DH complains that when he's serving soup, the container says it's, for example, 600g, but he puts 300g in the first bowl then there's only 260g for the other bowl.
Today I decided to push this as far as I could. Covent Garden Pumpkin soup. 560g. Now, it's a Tetra Pak, so tricky to get it all. I poured it out and shook it as much as I could. 520g. So I cut it open and used a spatula to scrape it all out. 553g
Technically, I suppose, we're close to 560g. But nobody who has this soup is going to do what I did and actually get 560g out of it.
I think producers should have to declare the usable mass under standard conditions (pour it out, perhaps give it a few shakes and a tap).
I don't think they should have to take account of the wastage that comes from pouring it into a pan and heating it, or what might get left behind in the bowl if you aren't a bread wiper. That would be silly.
But then, the other part of me thinks that producers don't have to account for the stalks on grapes, or the skin on onions... it's a minefield. I guess that's why we see people snapping the stalk off a broccoli before weighing it, which is probably why a lot of broccoli is wrapped in plastic these days.