Casting in Armor

[Archade]

So:

Thistledown Material Armor (ROW) -10% arcane spell failure
Mithral Material Armor (DMG) -10% arcane spell failure
Twighlight Magical Armor (PHB2) +1 equivalent, -10% arcane spell failure
Githyanki Battle Caster (MM4) -10% arcane spell failure
Spellsword (CA) 1st level -10% arcane spell failure, up to -30% arcane spell failure
Suel Arcanamach; (CA) up to -20% arcane spell failure

It would be pretty easy to build:

+1 twighlight mithral chain shirt (+5 AC, 0% spell failure, 5100 gp)
+1 twighlight mithral heavy steel shield (+3 AC, 0% spell failure, 4320 gp)

+8 AC for 10,000 gp. Without any special prestige classes or feats. If you go with 1 level of Spellsword, you could make the armor chainmail, but for a measly +1 more AC, why bother?

 

[Thurbane]

Actually, I have a related question: say a character is using a shield and armor, both of which have an arcane spell failure chance, are the two percentages added together and one check made per casting, or does the character make two checks - one each against the respective percentages?

 

[pawsplay]

Added.

Shields: If a character is wearing armor and using a shield, add the two numbers together to get a single arcane spell failure chance.