Arcane spell failure reduction for mithral--shield + body armor

[Hrafnkel]

This is a technical question and I'd like to make sure I'm not reading the rules wrong before I take the plunge with my character.

She's a sorceress with one dip into oracle, so she's proficient with most shields and also medium and light armor, playing in PFS organized play. I was considering getting her a mithril buckler and a mithril chain shirt after taking the arcane armor training feat at 5th level. This sounds to me like she'd have a 0% arcane spell failure chance:

Chain shirt = 20% arcane spell failure chance, -10% for mithril, -10% for arcane armor training = 0 arcane spell failure chance

Buckler = 5% arcane spell failure chance, -10% for mithril = 0 arcane spell failure chance.

What's throwing me a bit is the rule on page 150 of the CRB that says when you're using both a shield and armor, you add the spell failure chances together, in this case for 20% for the shirt and 5% for the shield, for a total spell failure chance of 25%. In that case, would you still be left with a 5% failure chance-- 25% for shirt + shield, -10% for arcane armor training -10% for mithril = 5% arcane spell failure chance.

I guess what it comes down to is this--is the -10 arcane spell failure chance for mithril, specified on pages 154-155 of the CRB, applied twice--once for the shield and once for the body armor, or is it applied once to the cumulative arcane spell failure chance of the shield plus the armor?

The more liberal reading makes sense to me, but as we all know, that often doesn't make much difference if you're sitting at a table with a more restrictive DM. I'd like get some idea of the consensus before I make the move with my character.

Thanks.

[Jeraa]

You figure the Arcane Spell Failure for each individual item first, then add those results up.

So a chain shirt has ASF of 20%. Making it from Mithral reduces this to 10%. Activating the Arcane Armor Training feat (which requires a swift action) reduces this further to 0%. So the final ASF of the mithral chain shirt is 10%, or 0% with the feat activated.

A buckler has an ASF of 5%. Making it from mithral reduces this to 0%.

Now that we know the ASF of each component, we add them up. The 10% from the mithral shirt, plus the 0% from the mithral buckler equals a total Arcane Spell Failure chance of 10%. (When the Arcane Armor Training feat is activated, this drops down to a total ASF chance of 0%.)

[Grick]

Mithral: "Spell failure chances for armors and shields made from mithral are decreased by 10%"

Table: Armor and Shields
Chain shirt - 20% Arcane Spell Failure Chance
Buckler - 5% Arcane Spell Failure Chance

Thus:

Mithral Chain shirt - 10% Arcane Spell Failure Chance
Mithral Buckler - 0% Arcane Spell Failure Chance

This is supported by an example item:

Mithral Heavy Shield: "This heavy shield is made of mithral and is thus much lighter than a standard steel shield. It has a 5% arcane spell failure chance and no armor check penalty."

Compared to a regular
Shield, heavy wooden - 15% Arcane Spell Failure Chance

So the reduction for Mithral applies to each item, because it's affecting the item itself.

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

Since you're wearing armor and using a shield, add the two numbers together.

10% + 0% = 10% ASF.

You don't apply the Mithral reduction to your total ASF, because that would make no sense. (IE: a mithral buckler and normal padded armor would end up as 5% + 5% then mithral -10% for no ASF, but dropping the buckler would suddenly make your armor more cumbersome.)