Blocks (and armor) generated by enchanted items
What makes defensive items a bit different is that you can use the defensive item to create a block (or armor) after you roll your defense. Also, since it's a non-action to use, you can use them to create defenses even when you are surprised, and have your defense reduced to 0 (plus the dice roll).
Example 1:
Harry is being shot at by goons. The goon rolls a 3 to hit. Harry rolls his athletics of 3 to dodge and gets (+, -, -, _) for a final result of 2. But since the goon is using an elephant gun (weapon: 4) Harry doesn't want to take the stress 5 hit. So he activates his magic duster (block 4) to completely defend against the goons attack.
Example 2:
Harry is walking around town, minding his own business when another goon takes a shot at him. The goon has the Lying in Wait aspect. The goon succeeds in surprising Harry, so Harry's defense is reduced to 0.
The goon rolls his attack 3 + (+, _, _, _) and tags Lying in Wait for a total of 6.
Harry rolls his defense 0 + (_, _, _, _) for a total of 0.
The goon is using a pistol (weapon: 2), so Harry would be looking at an 8 stress hit.
Harry uses his duster to generate a block 4, so reduces it to a stress 4 hit.
Note: Since the goons attack got though the magical block, it is broken so it no longer provides any protection to Harry. If, for example, there was a second goon who was also lying in wait, then Harry's defense would be back to 0 again. Of course, he could use his duster again to generate a second power 4 block, but it uses another charge.
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Stacking Enchanted Item defenses
There seems to be nothing stopping you from having 2 different defensive enchanted items, and using one to generate a block, and the other to generate armor both to apply to the same attack.
Example:
Steed has lent Harry his magical umbrella. This is normally a power 5 item, but since Steed didn't pay the extra points to allow it to be used by others, it acts as a power 4 item when Harry uses it.
Now, we'll flash back to the goon ambush from example 2, above.
Harry generates a block 4 with his duster, but he's still looking at a stress 4 hit. He then uses Steed's umbrella to generate armor 2, further reducing the attack to a stress 2 hit.
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Evocation Blocks and Duration
Evocation blocks normally last until the end of your next action. This means that you always have the chance to extend your block for more duration (Prolonging Spells, YS 259).
Blocks with more shifts put into duration are still broken as normal. So your long duration block can be brought down early by someone who rolls well.
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Reactive Evocation Blocks
The RAW (rules as written) don't seem to allow you to generate evocation blocks without spending an action. However, Harry seems to do just this several times in the novels. Most notably, he's able to generate blocks against things that are much faster than he is, and in game terms, would have initiative on him.
So, several GMs allow reactive block generation. Many choose to limit it in some way. Examples are:
have to use rote blocks
can't generate long duration blocks
can't generate blocks when surprised (i.e. when defense would be reduced to zero)
before the confrontation have to self apply an aspect, which can later be used to allow reactive block generation
etc.
Personally, I'm all for allowing reactive block generation. I think the real limit on spellcasting is mental stress, which you run out of soon enough.
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Modern Body armor
Modern body armor probably doesn't deserve a lower rating against melee attacks. Modern ballistic vests tend to also be highly stab resistant.
If allowing armor:3 mundane armor, don't allow such armor to be concealable. You can't really wear a class III or class IV vest under a coat and look normal. It's also reasonably heavy (20-30 pounds).