As I've been futzing with numbers, playing with DPS calculations for TBC talents and spells, I've been working on the following assumptions:

- Casting-side modifiers are additive. My only evidence for this at present is the character screen report when I have a succubus MD buff and Soul Link active. My damage is listed with a "+13%" modifier, which is correct for additive modifiers, but not for multiplicative modifiers. This becomes more of an issue with, say, TBC demonology talents that could be 1.1 + 1.03 + 1.05 = 1.18, or 1.1 * 1.03 * 1.05 = 1.18965.

- Target-side modifiers are multiplicative. IE, Curse of Elements + Fire Vulnerability = 1.1 * 1.15 = 1.265.

Does anyone know if these assumptions are correct?

Pretty sure it's multiplicative, from what i've seen on my damageshart while stacking enrage, deathwish etc, never had x145%, always.
and with deathwish + berserker buff im pretty sure it tells me x156%

Think you need to futz with the numbers a bit more.

MD buff = 10% buff
Soul Link = 3% Buff

1.1 X 1.03 = 1.133 Or a 13.3% damage bonus Rounded down, it would show you 13%

100% positive it's multiplicative, like every other % modifier in the game. It's easy to test on a level 60 hunter: spec for RWS and Slaying and go arcane shot a beast or humanoid.

Aye, but the character screen doesn't seem to round anything else down. Examples: attack power, attack speed, DPS, crit% in the spell book -- these all show numbers to at least 1 or 2 places. Why would the 13.3% be rounded down?

I started out assuming that these were multiplicative, just as the target-side damage modifiers are, but this anomaly makes me wonder.

They're absolutely multiplicative. If you don't believe it then spec Fire Power or Piercing Ice with Arcane Power. There's your proof.

OK, that's all good enough for me. Multiplicative it is. Thanks, all!