• block 30% more damage
  
• increase magic resistance by 12%

Would the block 30% more damage prevent 30% of a dragon's breath attack from harming me or is it only physical damage? Also does the magic resistance mean a 12% chance to resist the entire magic attack (in this case dragon breath) or just that 12% of that magic damage is ignored? If block 30% more damage is better than 12% magic resistance with dragon's breath does that also apply to normal spells that magic using NPCs would cast at me?

Can you give an example where the "30% damage blocked" is coming from?

12% magic resistance means magical spells are 12% weaker against you.

elven shield of eminent blocking has "block 30% more damage with your shield" and elven shield of waning magic has "increases Magic Resistance by 12%".

Ohhh, I understand now. That effect is for physical damage only. Blocking induces a damage reduction multiplier, which is further augmented by the "block more damage" enchantment. The magic resist enchantment is passive, and you don't actually have the shield up to reap the benefit; it just has to be equipped.

High block skill and just 1 perk in Shield Wall is all you really need, dont need the extra enchantment. If you wanna stop dragon breath, get the Spellbreaker. Spellbreaker is capable of completely negating a spell that hits it while your blocking. Some dragon breath may get through it tho, but not much.

Spellbreaker is the best against dragons and all kinds of necromancers. It doesn't consume your magicka like a ward spell. Later in the game you can make your own shield with stronger enchantment on it.