Attack is a 1:1 ratio with the stat that governs it. For my 50 Archer (http://imgur.com/fGo9d1u.jpg ) you can see that 312 Dex gives my 312 Attack Power.
I believe Potency is a multiplier. 130 would be x1.3. So as others have said 30% bonus to the normal amount.
The interesting thing about the 1:1 ratio is it leaves room for some versatility in the class system. Though, jobs are another story.
All damage and healing pass through the check of Determination. I'd like to see what that means for, say, a lancer who tosses on some mind gear to self cure. It obviously won't be as potent as a conjurer with all their traits and higher-level cures, but it'd be something.
Anyway, I believe that the Potency is a multiplier in play after your Attack Power passes through the Determination check.
I think that Attack Power is your maximum damage potential curve, and determination dictates where you land on that curve. Obviously it'll take quite a bit of testing to figure out, though.
Edited, Jun 25th 2013 7:21pm by Sephrick