Haste, Haste, Haste! How the frak is it calculated?

Most people add haste sources together, for an arcane mage the basic form is Netherwind Presence (+6% spell haste) + Haste Rating. Blizzard treats each source of haste independently otherwise once you get a raid going it gets out of control really quickly, also it means you get a little less benifit. Lets assume 10% from hast rating

The Wrong Way: 6% + 10% = 16% so Arcane Blast goes from 2.50 to 2.10 (2.50 x
(1-16%))

The Right Way: I will show all the steps here for clarity and try put the numbers into words

New Cast Time = base cast time divided by the product of the benefit of all haste sources

NCT = 2.50 / ((1 + 6%) x (1 + 10%))
NCT = 2.50/ (106% x 110%)
NCT = 2.50 / 116.60%
NCT = 2.14 seconds

Some care needs to be taken with this for example Hyperspeed Accelerators are increasing your haste rating not adding another source of haste like (for example) Heroism, Icy Veins, and a Wraith of Air Totem would. Just for kicks- if you popped Icy Veins right now it would look something like this

NCT = 2.50 / ((1 + 6%) x (1 + 10%) x (1 + 20%)
NCT = 2.50 / (106% x 110% x 120%)
NCT = 2.50 / 139.92%
NCT = 1.79 seconds

It can get very complicated during a boss fight with all the various sources interacting together. Your cast times will go all over the place!

So Now We Know How it Works, Lets Look at Arcane Numbers!

Arcanist's will have access to at least three pools of haste, Haste Rating, Netherwind Presence and Icy Veins, So lets have a quick look some cast times with these at different levels of Haste Rating. For simplicity I will use Arcane Blast as it will be the most common spam and top it off with a graph.

Base Casting Time 2.50 seconds

Haste Rating 0 (+ 0%)
+ Haste = 2.50
+ Haste + Netherwind Presence = 2.36
+ Haste + Netherwind Presence + Icy Veins = 1.97

Haste Rating 164 (+ 5%)
+ Haste = 2.38
+ Haste + Netherwind Presence = 2.25
+ Haste + Netherwind Presence + Icy Veins = 1.87

Haste Rating 328 (+ 10%)
+ Haste = 2.27
+ Haste + Netherwind Presence = 2.14
+ Haste + Netherwind Presence + Icy Veins = 1.79

Haste Rating 492 (+ 15%)
+ Haste = 2.17
+ Haste + Netherwind Presence = 2.05
+ Haste + Netherwind Presence + Icy Veins = 1.71

Haste Rating 656 (+ 20%)
+ Haste = 2.08
+ Haste + Netherwind Presence = 1.97
+ Haste + Netherwind Presence + Icy Veins = 1.64

Haste Rating 820 (+ 25%)
+ Haste = 2.00
+ Haste + Netherwind Presence = 1.89
+ Haste + Netherwind Presence + Icy Veins = 1.57

Blogger is still fighting with me a little... I am sure I will get the hange of it soon!