This table indicates for example, that you have a 50/50 chance of rolling 11+ on 3 dice.

The values in the table above were hand-calculated.  The table does not take into account miscasts, irresistible force, etc.
 

Ever wonder Geez, why can't I ever roll an irresistible force?  Or, why do I miscast all the time?
 
 
 
 

Odds of Irresistible Force / Miscast

	2 dice		3 dice		4 dice		5 dice
	Irresistible	Miscast	Irresistible	Miscast	Irresistible	Miscast	Irresistible	Miscast
Normal Wizard	2.8	2.8	7.4	7.4	12.7	13.2	17.9	19.6
Skaven	0.0	2.8	9.7	7.4	9.9	13.2	2.7	19.6
Buzgob, rerolling miscasts	2.9	0.1	8.0	0.6	14.4	1.7	21.3	3.9
Buzgob, rerolling if not IF	5.5	2.7	14.3	6.8	23.8	11.5	32.5	16.1
Balthazar Gelt	8.3	0.0	19.9	0.5	31.1	1.6	40.3	3.6
Book of Hoeth	11.1	2.8	29.6	7.4	50.9	13.2	71.3	19.6

Notes
The odds were calculated using a simple C program I wrote.  It generates 1,000,000 casting attempts per category, then shows the percentage of miscasts and irresistible force.  At first I started out calculating the odds by hand, but this got tedious.  The early hand-calculations were used to confirm that the program was working, at least for the case of a normal wizard.  The displayed percentages are rounded to the nearest tenth of a percent.

Skaven wizards, unlike everyone else, do not achieve irresistible force by rolling double sixes.  Instead they get irresistible force by rolling an unmodified total of 13.  At first I was led to believe that this was a good thing.  The table proves otherwise.  The only time a Skaven wizard comes out ahead is when rolling 3 dice, and then only by an additional 2.3 percent.

Buzgob refers to Buzgob's Knobbly Staff, a 50 pt arcane item for Orcs n Goblins.  Buzgob's staff allows the bearer to reroll all the casting dice once per magic phase.  Two basic strategies come to mind.  The first is to use the staff to avoid miscast, and throw lots of dice on a really dangerous spell like Waaagh.  The staff will reduce the chances of miscast while the odds of irresistible force go up due to the extra dice.  Another strategy is to try for irresistible force.  This is a riskier approach as it raises the odds of a miscast.  But it could be worth it -- if casting Waaagh will win the game, but your opponent has a dispell scroll.

Balthazar Gelt is just as dangerous as I suspected.  I always knew the little bastard was too good for his britches, but this clinches it.  See the discussion below.

Book of Hoeth is a 100 pt arcane item available to the High Elves.  A wizard carrying this book will get irresistible force on any doubles, not just double sixes.  However, rolling two or more 1's still counts as a miscast.  While the Book gets really high rates of IF with high dice, it also runs a higher risk of miscast.  I would say the Book really shines when throwing 3 dice.  Throwing irrestible force on 1 in 3 spells is nothing to sneeze at.  Also, with 3 dice per spell you can attempt more spells while minimizing miscasts.
 

Balthazar Gelt -- Underpriced?
Balthazar has the Staff of Volans, which provides him with what amounts to an unlimited supply of Dangly Wotnotz (a 25 pt Orc n Goblin Arcane item).  He can use the Staff of Volans every time he tries to cast or dispell.  Typically he will use this ability 6 or more times per game.  Based on this we could estimate the value of the Staff of Volans at 150 pts.  Or we could compare the staff to the Book of Hoeth.  While The Book gets irresistible force a bit more often than The Staff, The Staff almost never miscasts.  Balthazar can throw 5 dice with nearly zero risk of a miscast.  I would therefore rate the Staff a little higher than the book.  Given that High Elf magic items are given a 10-30% discount to play-balance the puny race, The Book of Hoeth has an equivalent cost of 120 pts.  This would put the Staff of Volans around 145 points.

Whew.  What else does Balthazar get?  Alchemist Supreme -- grants two extra spells: each worth 15 pts each (this ability is twice as good as the identical Necrarch ability Forbidden Lore, which costs 15 pts.)  Al-Kahest -- like a throwing knife that doesn't allow armor saves; let's say 15 pts (the cost of giving a longbow/handgun to an Elector Count).  Panacea Universalis -- automatically heals one of his/pegasus wounds without having to cast a spell, lets say 25 pts.  The Gleaming Robe -- a 3+ Ward save vs magic/missiles.  I would say 50 pts for this, because it protects his mount as well, which is fairly unusual.  Amulet of Sea Gold -- Magic Resistance 1 vs enemy spells, this is worth 20 pts (identical to the Sigil of Sigmar.)

Balthazar Gelt's cost breakdown:
Wizard Lord              175
Level 4 upgrade          35
Pegasus                  50
Alchemist Supreme        30
Al-Kahest                15
Panacea Universalis      25
Staff of Volans          150
Gleaming Robe            50
Amulet of Sea Gold       20
Total Estimated Worth    550 points
Actual Cost              450 points

Yeah, he's a pretty good bargain at 20% off.  Do I have an axe to grind or what....

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