Sure...whatever Chair said. Arcane Missles is a channeled spell so there is no comparison between using it as a channeled spell or using it as a casted spell since it simply cannot be casted.

As for shield block? Ugh...you're really making me look that up aren't you? Feh...

Alright, after a little bit of research here's what I found:

All nonelite mobs have a weapon skill equal to level x5 and a defense skill of level x5. Elite mobs and bosses can have a buff to those in addition to increased damage or armor. If your defense skill is equal to their weapon skill then you will see only basic returns of 5% crit chance, 0% crushing blow chance, etc etc etc. For each +1 defense on a level 60 warrior with 300 defense against a level 60 NPC with 300 weapon skill you will see +0.04% Block, dodge, and parry. That means for every 25 defense you will increase the chance of a block/dodge/parry against someone using 300 weapon skill (including PCs I suppose) by 1%.

The opposite is true as well. If your defense is lower than their weapon skill then you will increase their chance at making hits and critical hits. If you are fighting an NPC with 15 or more weapon skill over your defense skill (think bosses or level 63 elites with weapon modifier) then you have a 15% chance to take a crushing blow. This percent increases by 2% each additional point you are lower than 15. So 300 defense against a 315 mob will see a 15% chance to take a crushing blow, but 290 vs 315 will see a 35% chance.

Moving along we have to consider the armor value of the shield. The amount of damage reduction from armor is curved and found by using the following formula:

% Reduction = (Armor / ( Armor + 400 + 85 * EnemyLevel)) * 100

Your character screen armor reduction, BTW, is based on the formula against a level 60 nonelite mob. Armor reduction is capped at 75% so you cannot exceed that. So the amount of damage reduced by the armor on your shield is the difference in armor reduction between using it and not using it.

As for the actual 'block' it is a combination of all those above and more.

The formula for the damage reduced by a block is as follows:

X = [(Shield Block Value] + ((Strenght / 20) - 1)]

If you are a paladin with shield spec then you use:

X = [(Shield Block Value * 1.Y) + (Strength / 20) - 1)]
Y = Amount from talent so 10% is 1.10 and 30% is 1.30

Now the check for block is done after the check for a hit and BEFORE the check for a crushing blow and critical strike. That means if you block an attack you cannot recieve a critical strike or crushing blow.

A 25% chance to block is 25% chance to block X damage and 25% chance to turn a critical hit into a blocked hit.