Priority based rotation

A priority based rotation is a system that players use to maximize their DPS based on the worth of their abilities. The worth of a spell or ability is commonly associated with its Average Damage Per Cast Time (aDPCT). With Cataclysm and the recent changes associated with Patch_4.0.1, most players are relearning their classes and how best to put to use their new or reworked abilities. The goal is to learn how to calculate the average damage per cast time (aDPCT) of our spells and put together a priority-based rotation, not only helping our understanding and appreciation of the design process, but also (and perhaps most importantly) maximizing our DPS.

Coefficients
When designing a DPS rotation the best place to start is with the abilities/spells themselves. The coefficient decides how much of your increasing power (either spellpower or attack power) gets applied to any given ability's overall damage production.

For example, a Warlock's Shadowbolt spell has a spellpower coefficient of 85% roughly. To calculate how much extra damage you'd receive from your spellpower, you can do the following, assuming you have 1000 spellpower:

New Damage = BaseAverageDamage + (CurrentSpellpower * Coefficient)

Substituting all the number inside, you'd get the following:

New Damage = ((640+714)/2)+(1000*0.85) New Damage = 677+850 New Damage = 1527

Resulting in Shadowbolt doing approximate 1527 damage now, originally doing 677 damage, but now receiving an extra 850 damage from spellpower. Coefficient tables for most abilities can be found at Spell_power_coefficient for ease of use.

Average Damage Per Cast Time
The aDPCT value, when calculated properly and thoroughly, is the most important step in conjuring a DPS rotation. The more information you have for your abilities the better! You must consider a spell’s coefficient and base damage, its specific critical strike percentage, haste rating, mastery, talent tree bonuses, glyphs, talents, and spell procs, etc. All of these things can be quite a daunting task to understand. But with proper logic it's not nearly impossible.

The formulate for calculating aDPCT is: aDPCT = [(bDMG + (pwr*Coeff))*(1 + (crit_mult*Crit%))*(1 + Mastery%)*(1 + treebonus%)] / (bCastTime / (1 + haste%))

Whereas:

aDPCT = Average Damage Per Cast Time bDMG = Base Damage of Spell (low end + high end / 2) pwr = Your current spellpower/attackpower Coeff = Abilities coefficient multiplier crit_mult = Critical Strike Damage multiplier (Melee have 200%, or 2, Mages and warlocks now have 1.995, or 199.5% increase with Patch_4.0.1 Crit% = Critical Strike % in decimal form (20% = 0.20)  Mastery% = Bonus Dmg from Mastery in decimal form.  treebonus% = Bonus Dmg from Talent Tree choices in decimal form.  bCastTime = Abilities Base Casting time. (Instant cast abilities use Global_Cooldown)  haste% = Haste % in decimal form.

An example, again using a Warlock's Shadowbolt spell, with 1000 spellpower, 15% critical strike, 15% haste rating, and 0% bonuses for mastery and talent trees (for simplicity.) aDPCT = ((677 + (1000*0.85)*(1+(1.995*0.15) / ((3.0 / (1+.15)) Simplified: aDPCT = ((1527)*(1.29925)) / (2.6086) aDPCT = (1983.95) / (2.6086) aDPCT = 760.5420

And now a spell to compare it to; a Destruction Warlocks Chaos bolt: aDPCT = ((1691 + (1000*0.71)*(1+(1.995*0.15) / (2.5 / (1+0.15)) aDPCT = ((2401)*(1.29925)) / (2.1739)  aDPCT = (3119.49) / (2.1739)  aDPCT = 1434.9782

Conclusion of Examples
In comparison, Chaos Bolt has an higher average damage per cast time, resulting in the player wanting to use it before Shadowbolt in all cases. (Of course the cooldown of the spell will be taken into consideration, but in this scenario you would keep Chaos bolt consistently on cooldown.