How to estimate importance of multistrike for arcane mage?

Icyveins gives the following stat priority for an Arcane Mage:

1. Intellect;
2. Mastery;
3. Haste;
4. Multistrike;
5. Critical Strike;
6. Versatility.

In order to understand this I took my 100lvl arcane mage and estimated damage (dmg) increase for each stat (relative to the dmg you would have if this stat = 0%):

1. +2.00% dmg for each +100 Intellect (if you have 5000 spell power)
2. +1.71% dmg for each +100 Mastery
3. +1.11% dmg for each +100 Haste
4. +0.45% dmg for each +100 Multistrike (+1.51% chance for 2nd attack)
5. +0.91% dmg for each +100 Critical Strike
6. +0.77% dmg for each +100 Versatility (+0.35% armor)

Here priorities for all stats but Multistrike become obvious.

As far as I understand, multistrike is important because it doubles the chanse to get charge of Arcane missiles. But how to estimate importance of this chance and why one should put multistrike after haste and before critical strike?

Furthermore, when you upgrade your mage state priorities might change. For example, if you have 71% Mastery and only 5% Haste, then for +100 mastery you would get 1.71%/1.71 = 1.00% of dmg increase, and for each +100 haste you get 1.11%/1.05 = 1.06%, so haste becomes more important and I can estimate the moment when it does. But how to estimate the moment when multistrike becomes more (or less) important than some other stat?