When is Random set?
Magic Find is calculated when the creature is killed.
Dropped loot is calculated when the creature is killed (taking magic find into account), and I remember a dev post (might have been during one of the betas, which are now deleted) saying that the contents of bags, boxes and the like are calculated the moment you open them.
wrapped up in some crazy ritualist hoo-ha from Cantha.
A real grab bag of ‘you can’t hurt me. They’re called Guardians.
https://forum-en.gw2archive.eu/forum/game/dungeons/Magic-Find-Chests/597487
They only work on creature kills, chests however have a better table then normal kills. We wanted to avoid the game play of killing a boss and then having to equip your whole Magic Find set before opening the chest.
Also magic find is calculated from when the creature is killed so it’s also not viable to use your normal gear set kill a boss then switch to magic find.
https://forum-en.gw2archive.eu/forum/game/dungeons/Magic-Find-Chests/598159
It does not work on the Mystic Forge, magic find only effects the loot generated from killing monsters. (this includes crafting materials, higher tiers of loot like rares & exotics… etc… )
Granted Zomoross is mighty and I’m sure he randomly gives more favor to those who are wearing a tin foil hat in real life and are spinning in their chair while clicking the forge button he is evil like that.
The second post also implies MF has no effect on containers like the champion loot boxes.