Diablo – Can Frost Nova with Shatter result in more than two Frost Novas

No, there is no such option in Diablo III.

(Blizzard decided against it after they found out that during play-testing, players tended to switch to Magic Find gear just before landing the final hit on a boss :)

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For the Critical Mass passive, what is meant by the "chance" is that it is determined by the proc coefficient of the spell you cast. Single target spells have 100% "chance" to reduce the cooldown by 1 second while AOE spells have differing sets of proc coefficents by spell AND rune. There's a few sites that break down proc coefficents of various spells which applies to crit chance as well as return on LOH. Here, and here. Luckily people with more patience than me seem to have most spells crit proc chance figured out.

Some useful proc chances for popular wizard spells:

Hydra: 0%, Hydras never proc
Magic Missile / Seeker: 100%
Shock Pulse / Piercing Orb: 25% per target hit
Blizzard / Snowbound: 2% per tick per target hit
Energy Twister / Wicked Wind: 10% per tick per target 
Explosive Blast / Chain Reaction: 11% per target hit per explosive (of 3)

I've recently been trying the Windup Wizard/CM build which still seems pretty effective, and a bit silly (in fact it was the easiest way for me to beat inferno Ghom). From personal experience I've found that on my wizard, when I run over 40% crit chance while using a build like windup wizard, everything is off cooldown pretty much all the time.