But yeah... Doing testing with Arcane Dynamo, Critical Mass, Magic Weapon Shocking, and +AP on Crit, these numbers are all used there:
For Arcane Dynamo each hit has a probability the same as the modifier, each calculated independently... so for Magic Missile with Split, each hit has approximately a 33% chance to proc Arcane Dynamo. This means you can get anywhere from 0 to 3 procs on a cast, averaging one.
The same applies to Magic Weapon Shocking, and Critical mass (Hence Living lightning and blizzard sucking for Critical mass, and meteor and arcane mines being pretty good).
Lastly for AP on Crit, the AP gain is multiplied by this value... so if you have 10 AP on Crit, and crit with Shock Orb, you'll get 2.5 AP... and it does appear to use and consider fractional values or adjusts the probability to compensate... since Living lightning works with it.
Also, a small correction, meteor is actually 25% on hit, and 25% from dot ticks, but the first dot tick is immediate, leading to seeing 50% on hit... if you try meteor on a target weak enough that the first hit kills it you'll consistently see the 25%.
variations on this lead to thinking that overkill hurts you. It doesn't... but certain spells, like meteor, and the beams (ray of frost, disintegrate) that tick very quickly and gain a small value per tick make it appear that it does...
You can actually measure tick size on them pretty easily by hit really low level enemies with them and see the damage.