Assume I'm playing a mage with Magic 6, Spellcasting 6, and have a Power Focus 6 along with a Sustaining Focus 6.

If I cast a spell at Force 6, then I can lock it into the Sustaining Focus 6 with no problem - it can handle up to Force 6 spells.

BUT...if I cast that Force 6 spell and use my Power Focus 6, then I believe it effectively makes that a Force 12 spell, which would then be beyond the ability of my Sustaining Focus 6 to handle.

Am I interpreting this scenario correctly? I don't normally use Power Foci for my characters, but this is coming into play with another post I'm preparing.

Power Foci DO NOT ADD FORCE to the Spell... They ONLY add dice... A Power Focus does not raise the Magic rating of the Caster it is bonded too...

So your Spellcasting is Boosted through the Dice Pool, but not via Force or Magic Rating. Still... Power Foci are very useful because they cover all the augmenting Foci except Weapon/Qi/Sustaining in effect (ie. they increase relevant Dice Pools that use the Magic Rating of the Character by the rating of the Power Focus).

Example:
Magic 6 + Spellcasting 6 + Power Focus 6 casting a Force 6 Spell and locking into a Force 6 Sustaining Focus.
You have 18 Dice to cast your Spell that Limits at Force (unless you Use Edge or Reagents to set/bypass Limits). Even if you Set/Bypass Limits and obtain 3/15 Hits on your spell, you can still Sustain it in your Rating 6 Sustaining Focus.

power focus does not increase the force of a spell.

Thanks for the clarification, as this part from the SR4A Core Rules, p. 200 seemed to me to indicate otherwise. Under "Power Foci" was: "A power focus adds its Force to all tests in which the magician’s Magic is included."

That wording says to me that Force 6 + an extra Force 6 = Force 12, so I appreciate the clarification.

yeah the wording could have been better, saying Dice equal to Force being added.

Well, the rulebook uses the confusing phrase "increase your effective Magic rating". But what that means is really only what it says in the next sentence: It adds to a DP whenever your Magic attribute is part of that DP.

To be fair... You are correct.