Calculation of the MP5 value of Haste Rating was a bit trickier and I feel the assumptions made are a little less robust than for mana pool, mana regen, etc.
- Flash Heal is being spammed (1887 base heal, 80.86% co-efficient and 1.5s cast time)
- Discipline has 1950 spell power
- Holy has 2147 spell power
- 32.8 Haste Rating equals 1% Haste
- Caster currently has zero haste. The effect of moving from 0% to 1% haste is slightly greater than moving from 6% to 7% haste. However the effect on weighting is very small.
- 30% over-heal
- Borrowed Time and Serendipity have been ignored as only FH is being spammed.
- Latency and Surge of Light procs are ignored.
Firstly we calculate the MP5 value of a Flash Heal. We calculate this so we can figure out how much an additional FH, enabled by haste, is worth.
Flash Heal MP5 value = (base heal + (SP * SP co-efficient)) * SP MP5 value * (1 – overheal %)
Flash Heal MP5 value is 1453.3 for discipline and 1520 for Holy.
Secondly we need to calculate the number of additional casts 1 Haste Rating allows.
Cast time for 1% Haste = Base Casting Time/(1 + (% Spell Haste / 100))
Cast time for 1% Haste = 1.5/(1 + (1 / 100))
Cast time for 1% Haste = 1.48149
Extra casts for 1% Haste = 1.5 – Caste time for 1% Haste
Extra casts for 1% Haste = 1 .5- 1.48149
Extra casts for 1% Haste = 0.014851
Extra casts for 1% Haste (%) = Extra casts for 1% Haste/ Base cast time
Extra casts for 1% Haste (%) = 0.014851 / 1.5
Extra casts for 1% Haste (%) = 0.990099%
Extra casts for 1 Haste Rating = Extra casts for 1% Haste / Haste Rating per Haste
Extra casts for 1 Haste Rating = 0.009901 / 32.8
Extra casts for 1 Haste Rating = 0.030186%
So for each 1 Haste Rating you get 0.030186% additional casts
Finally we bring the parts together to calculate how much MP5 value 1 Haste Rating brings.
Haste Rating MP5 value = FH MP5 value per second * Extra casts for 1 Haste Rating
Haste Rating MP5 value (Disc) = 1453.3 / 1.5 * 0.030186%
Haste Rating MP5 value (Disc) = 0.292463
Haste Rating MP5 value (Holy) = 1520 / 1.5 * 0.030186%
Haste Rating MP5 value (Holy) = 0.305889
I’m pretty sure this section could use some refinement and include perhaps latency at least. I generally play with latency around 300ms. This turns a 1.5s Flash Heal into a 1.8s Flash Heal. A very quick and dirty sensitivity test shows that dropping a 1.8s cast time into the equation results in a Haste Rating MP5 value of 0.243666 for Discipline and 0.254898 for Holy. The reason for this is not that the percentage of additional casts changes (it doesn’t), but that the same healing payload, and therefore equivalent MP5 value, is delivered over a longer timeframe. You get less bang for your buck. Or rather the same bang but for more bucks.
Incidentally the values for spamming Greater heal with 300ms latency are 0.321005 for Discipline and 0.335357 for Holy.