Is it calculated when a spell/projectile is cast or when a monster dies?

For example something slow moving like Tornado or Hammers. If you cast it with your primary setup, the projectile is traveling and then you switch to your MF weapons and the mob dies from it, is the MF calculated from the MF of when it was cast or when the mob died?

When it dies.

It's an interesting question, but surely you wouldn't ever bother to cast a bunch of hammers, bone spirits, or the like with a weapon such as hoto before switching to something like a gull before they hit?

If it was last hit on a boss you could do this every time you try.

Blizzard, bone spirit, Blessed hammer, guided arrow, Traps and some others im sure would be easy to jut click and then press W. Have all the damage on 1 side and the MF on the switch sword/shield side (could total up to easy extra 280 % MF with isted sword/shield)

ever killed diablo with a hdin or blizz sorc?

No one knows