The Enigma Order

A while ago there was a discussion between me and some of our priests on the guild chat about how Shadow Word: Pain damage was calculated. It sure is a fairly complicated matter and - as far as I can say - much unlike any other class. According to my best knowledge, this is how it goes (copy paste + some personal additions):

Basically SW:P will take into account any buffs you had at the time of casting and it will keep assuming those buffs as long as you keep it up with MF, with the exception of spellpower... any spellpower gains (or losses) are refreshed with MF... no other buff is however. This is true if you cast SW:P before a buff is applied, eg Earth and Moon (+5% crit for caster comming form a boomkin)... the MF refresh will not update your SW:P with the fact Earth & Moon is now on the boss, instead you have to recast the spell.

If you gain 200sp (say, from a trinket) for 20sec and then cast SW:P, SW:P will then tick for an amount that includes that 200sp. When you lose that 200sp SW:P will continue to tick as if you had that 200sp, however the next time you cast MF SW:P will refresh with your current SP and thus it will lose that 200sp temp gain.

If you gain 5% crit for 20sec and cast SW:P it will calculate dot crits inclusive of that 5% crit. When you lose the 5% crit after 20sec SW:P will continue to calculate crits as if you still had the 5% crit. When you cast MF the refresh is unable to recalculate your crit rate and thus SW:P will stay up on the target assuming you still have that 5% crit rate.

TLDR: aim at having all +crit effects on the time you cast your single SW:P, this additional crit will keep on doing extra damage through the whole fight (i.e. after MF refresh). The spell power you have at the time of application of SW:P practically doesn't matter.

Future reference - granted this remains unchanged come 3.3, the same logic which now goes for crit will apply to haste.