I'm not sure if this is a bug or how it's supposed to work but I was using the Crystal of Potential II on my mage and noticed it only takes into account the base spell damage without focus.
Was using Thunder of Ice (5500 dmg) + True Frost focus(80%, +4400 damage) for a total of 9900 damage. Power Potential II was only upping that to 15400, not the 19800 I thought it would be.
If that's the way it's supposed to be then cool.