Single dot ( innerfire only ) 907
Single dot when 5 stacks are up 998
Single dot applied in rotation 926 / 925
Single dot reaplyed after rotation 1010
Edit: – I should probably explain this post eh…
I was testing how to get the most out of Shadow Word Pain on a dummy. I wanted to see the differences in the ticking damage as to what difference it made when you added the dot.
and I found no joke that if you reapplied the dot ( So while you had 5 stacks of shadow weaving up, but had been casting that you got an extra +12 damage per dot. Which doesn’t make sence.
Provided by 
Pugnacious Priest's Warcraft Blog 
OK you have just uncovered the tip of the iceberg there. The bug is actually not that complicated and can be abused somewhat : you know how when you have a spellpower proc (like sundial, or using the ZA trinket) your already-casted SW:P ticks go up ? No you don’t have to recast it, slap SW:P on a dummy, pop your trinket and watch the tick value go up.
OK now there are other things that affect you’re SW:P tick values, namely Shadow weaving and Crit rating (through the Shadowform buff). The bug is that when those are updated, the SW:P ticks are not updated with the new values, you have to manually refresh it to take it into account. It’s actually true with all other dots, not just SW:P, the difference is that SW:P does not get recasts mid fights (because MF refreshes it).
Remember that the difference can actually be quite huge, in a well composed raid the difference between a well timed SW:P cast or refresh and a bad one can be upward of 28%. In specific fight (like Thaddius or Heigan) where your crit rating gets buffed up the wazoo it’s even more noticeable.
I have an add-on that tracks this (ShadowGreenLight, available on Curse / Wowace / WowI )