I have finally battled this windmill into submission.

The short version:  For ALL casters, it takes 80 (eighty!) Int for 1% spell crit.  The only difference is where each class starts.

There was a blue post a while back that explained that the base spell crit chance is 5%.  This is modified by the difference between the character's Int and the "expected" Int for that level applied at some mystical ratio.  The infamous 59.5 Mage number came from this at level 60.  There are other modifiers for target level difference, etc., but the fundamental one we all look for is "How much spell crit will this much Int give me?"

Through the magic of The Armory™, I have derived the formula:

The basic equation is: Spell Crit % = 5 + (Int - expected Int)/80

At level 70, the expected Int values are as follows:
Druid    252
Mage     327
Paladin  133
Priest   301
Shaman   224
Warlock  273

Go ahead, run your own numbers, you'll see.

I have not tested if the 80 number is actually level +10, but I would not bet against it.  Determination of the expected values to verify this would be quite time consuming.

For the purists that must have the spell crit % at level 70 in y=mx+b format (1/80 = .0125):
Druid    .0125*Int + 1.85
Mage     .0125*Int + 0.91
Paladin  .0125*Int + 3.34
Priest   .0125*Int + 1.24
Shaman   .0125*Int + 2.20
Warlock  .0125*Int + 1.59

Logic would dictate that Melee crit rate works the same way, but so far the numbers refuse to behave.

In a similar vein, for all classes, 22 crit rating = 1% crit.

Great work Dwarphro!

I have always been a fan of the numbers side of this game.  Perhaps I can illuminate your problem with melee to a certain extent. Melee has many different modifiers to it that spells do not have (or at least appear not to have).  Melee attacks can be dodged, parried, blocked, hit, crit or miss.  Spell can hit, crit, "white" resist, or "yellow" resist.   I know defense values have an effect on crits as well (or they used to).  So that complicates things too.  Good luck though, and keep up the great work!

80 is not level + 10