Do Power Foci's increase to Magic attribute apply to Spell Design? I know it does not say it applies in the BBB, but in MitS I did not see it stated the increase did not apply.

MitS does state power foci do not add dice to Enchanting tests. Which makes me ask, do power foci add to magic attribute for enchanting tests?

The force of a Power Focus is added to your Magic Attribute for purposes of determing if drain is Physical or Stun and the maximum number of spirits you can summon. The dice can/should be used as Spell Design is a function of Sorcery (albeit the Sorcery Background Skill).

So to answer your questions, yes it does give its dice for Spell Design, but no it doesn't improve Magic or give dice for Enchanting.

I was looking at TNs.

During Spell Design, Magic attribute is a negative modifier to the TN. When making orichalcum, Magic attribute is a negative TN modifer.

As you said, power foci can add rating and dice to a mechanic.

It specifically states when it's used in that fashion, and it only applies to your Magic Attribute for purposes of drain and number of spirits. The bonus dice are the only possibly benefit during spell design as you can rationalize it as a use of Sorcery.

Like the Doc said, so it is.

Just don't forget that if you use Sorcery (and thus gain the bonus dice from your Power Focus) you're at a +2 TN for defaulting to something other than the Spell Design skill.

Sphynx

Aren't you also limited in using Pool dice if you are defaulting?

half i think, if it's a speacilization.