Originally Posted by Arelenda

Testing proves:

for nukes: (Shadow Bolt, Soul Fire, etc)
Spellpower AND Multiplier are measured at the COMPLETION of cast.
If it's a projectile, it'll start traveling at that point.

for dots (Corruption, UA, etc)
Spellpower "locked in" at completion of cast.
Multiplier reevaluated per tic

for channeled spells: (Drain Life, Hellfire)
Spellpower is "locked" at START of cast.
Multiplier is recalculated at every tic.

I'm assuming crit bonuses (frost mages vs frozen targets) are also "locked in" at completion of cast, but am unable to test this.

I'll need to update ShadowSeer.