Good on ya Twird for deciding to do the maths full on, but correct me if I'm wrong, a simple formula for just DPS would be:
X = (Remaining DoT time/Entire DoT time) * DoT mana cost
Y= (1.5/Entire DoT time) * DoT Damage
Z= 3/5 of SB Damage OR Dps of another DoT * 1.5 (Y)
((Dmg of DoT + (135 * Co-efficient)) / DoT Duration - (Dmg of DoT / DoT Duration)
- ((Mana cost of DoT-(X)) / Mana gained from LifeTap) * Z
- (DoT Time Remaining/DoT Duration) * Z
- ((3 - Time till DoT tick) * Original DoT dps) / DoT Duration
=Change in DPS =)
The first calculates the increase of DPS for the DoT that is reapplied while the proc is up.
The second line calculates how much dps you are losing by using up mana, and therefore having to use up the GCD for another LifeTap/Darkpact.
e.g If the spell cost 400 mana, and you get 800 back from LifeTap/Darkpact, the DoT would then be costing an extra .5 of a lifetap/darkpact GCD (.75 sec), therefore 3/10 of a shadowbolt or 1.5 of a DoT's DPS.
The Z bit is just the damage that could have been done during the GCD.
The X bit calculates how much mana you are really losing, as you would have refreshed the DoT in a bit anyway, so the mana loss only occurs from the part of the DoT still ticking.
e.g Corruption costs "500" mana, but you have five seconds to go. Therefore the amount of mana that you are losing is (5/18 x 500) giving a total loss of 138 mana. This makes the example for the one I used above wrong! But its simple so I'm not gonna change it.
The third line is the DPS lost to a GCD. As you are not going to use up an entire GCD due to having to refresh the DoT after it expires, this is just the fraction of the old DoT that is wasted.
E.g You have 6 seconds left on a 18 seconds spell. That would equal one third of a GCD because if you do this three times, thats 18 seconds of that DoT that was 'wasted', equaling a GCD.
The fourth line is the DPS lost due to having the previous DoT closer to ticking than the new one.
You also have to take in the fact that refreshing the DoT before time costs extra resources compared to relatively little gain, a resist would decrease your DPS even more + use up more resources. However this might be slightly beneficial in a fight where you are stunned a lot, as that way you have less time with no DoTs ticking.
Personally, I wouldn't bother refreshing an existing DoT as the proc is 15 seconds long, so chances are your DoTs would run out during its duration anyway. I'd rather Shadowbolt, as that can crit and also activiate ISB (something else that should be included in calculations, but I can't be bothered).
Also, Agony should always be allowed to run to the full as it does most of its damage at the end.
Co-efficients Have fun!
Edit: Boy did I stuff up the first time >.>
Conclusion : After doing a couple of short calcs, the DPS lost to GCD and mana induced GCD is more than 100x greater than the dps increase caused by that proc.
e.g
135/18 = 8
3/5 of a shadowbolt= ~1200
0.o Massive as stated by Jenovaomega unless you only have a very few seconds remaining, and the proc is about to run out aswell.
Disclaimer: Its four o'clock in the morning and I just woke up....... ^.^
Edited, May 14th 2008 8:58pm by germa