Calculating SW:P Damage

[heriana]

So, I'm curious as to how exactly to go about calculating spell damage. My main issue here is which talents/buffs are additive and which compound upon others.

The talents, glyphs, passive buffs, etc that affect SW:P while solo are as follows:

Shadow Power: 15%
Empowered Shadows: (Mastery Based)

Improved Shadow Word: Pain: 6%
Twisted Faith: 2%
Shadow Form: 15%
Twin Disciplines: 6%
Dark Evangelism: 10%

Glyph of Shadow Word: Pain: 10%

Now, I know [Base Damage + (Spell coefficient x Spell Power)] gives us the very basic equation. How do you go bout accounting for all of the other 'buffs'. I know you don't multiply by the product of them, nor multiply by the sum of them. Essentially, which are additive and which compound?