Skill Vs Attributes

I've been seeing a lot of threads talking about BT/VD/etc sets, and I was wondering what determines a switch from Skill-based gear for physical spells to attribute-based sets (I'm mostly thinking of Loki's vs Magus). I personally don't know the formula for calculating Skill to Damage, so if a number cruncher could help a dood out, I'd be much obliged.

http://wiki.ffxiclopedia.org/wiki/Calculating_Blue_Magic_Damage

D is calculated as floor(Blue Magic Skill*0.11)*2 + 3

Here is the page for calculating base D of spells. I do recall each spell having a cap, but for newer spells I can't imagine we have enough skill to hit it just yet. For almost any physical spell go w/ your AF+1 body.