Often modifiers (haste, damage and so on) will be referred to by bonus percent. You always multiply the normal 100% with this bonus. For instance:
Adrenaline rush increases your energy regeneration by 100%
This means that your rate of energy regeneration is increased by itself (rate + rate). Therefore, plus 100% means double - as it stands, Rogues regain:
20 Energy / 2 Seconds
so Adrenaline rush would cause you to regain:
20 + 20 Energy / 2 Seconds = 40 Energy / 2 Seconds
The wording on abilities may vary. Sometimes a modifier will be referred to as adds 50% damage, which means the damage is now 150% times that of the original. In other cases, the wording is like in the example above; increases your energy regeneration by 100%, meaning the energy is twice that of the original rate.
In general, the following terms are equivalent:
- Ground speed increased by 60%
= You move at 160% run speed.
- Adds 25% critical strike bonus damage
= Your critical strikes will do 1.25 times as much damage.
- Increases the duration of your ability by 15%
= Your ability now lasts 1.15 times as long.
It is a common mistake to read "by X%" as "to X%". When a value is increased by a percentage, that means you take the norm (100%), and further increase it by the bonus. Mount speed works this way; for example, +310% epic flight means your run speed becomes 100 plus 310 percent, ergo you move >4 times faster than a normal running player character.
When a value is increased to a percentage, then you take the quoted percentage. But this wording usually doesn't happen in World of Warcraft.
As of Patch 1.12, all modifiers have been changed to multiplicative (as opposed to additive). This means that an effect of +30% and an effect of +50% would not equal an effect of +80%, but instead an effect of +95%.